[[Embryology and Obstetric Anatomy]]
First trimester scans should include:
- Search for gestational sac, yolk sac and embryo.
- Presence of cardiac activity
- Foetal number
- Maternal anatomy including fibroids, corpus luteum, adnexal masses (surgical removal usually in 2nd T if required), free fluid (only a small amount of anechoic fluid should be present).
Second and third trimester scans should include:
- Foetal presentation
- Amniotic fluid volume subjective and quantitative assessment (AFI or deepest pocket in twins).
- Placental location, apperance, relationship to internal cervical os.
- Umbilical cord insertion and number of vessels.
- Estimation foetal weight and relation to gestational age
- Foetal anatomic survey after 18/40
- Head – Falx, cavum septum pellucidum, skull ‘bone’s, atria of lateral ventricles (tilting slightly caudad from standard axial, </=10mm), choroid plexus (fills 60-90% of atrium), cerebellum (vermis echogenic, axial measurement), nuchal thickness, cisterna magna (anechoic posterior to vermis, normal 2-10mm)
- Face – Orbits, nose, jaw, lips, profile
- Heart – position, axis, 4 chambers, intraventricular septum, foramen ovale, mitral valve, tricuspid valve.
- Great Vessels – LVOT, RVOT, aortic arch, ductal arch
- Abdomen – Situs, stomach, kidneys, bladder, abdominal wall
- Spine – Ossification centres (vetebral bodies, laminae), skin line. ‘Sagittal’ image is actually oblique along body and lamina. Axial images required to exclude meningomyelocoele.
- Extremities – long bones, hands/fingers, feet/toes, position of joints
- Gender when medically indicated
- Maternal anatomy including search for adnexal masses, fibroids, cervical length and shape.
Foetal Measurements and Growth
Pregnancies dated from 1st day of last menstrual period (LMP). Gestational age (GA, clinical) = menstrual/embryonic/foetal age, based on 28/7 cycle, conception at 14/7. Term 37-42/40. Symphysis-fundal height (SFH) is clinical estimate of foetal size, measured from symphysis to fundus, should be GA in weeks ± 3cm, less accurate in later pregnancy. LMP used if EDD within 5/7 of US age; otherwise US EDD used. GA is also approximately equal to transverse size of cerebellum and renal length in mm.
- Gestational sac size measured in 3 orthogonal planes = mean sac diameter (MSD). Used when embryo not visible, accurate to within ~1/52. GA (days) = MSD + 25.
- Crown-rump length (CRL) from top of head to bottom of torso, useful until 10-12/40 (when other measurements more accurate). GA (days) = CRL + 42. Accurate to within 3-4/7.
- Biparietal diameter (BPD) from axial head at level of 3rd ventricle, thalamus and cavum septum pellucidum (cerebellar hemispheres should not be visible), from outer table near cranium to inner table far cranium. Inaccurate if dolichocephaly (elongated skull) or brachycephaly (round skull). Generally GA = 4 x BPD (cm) + 1 (~50mm at 20/40, ~100mm at 40/40).
- Head circumference (HC) is outer perimeter at same plane as BPD.
- Abdominal circumference (AC) is outer perimeter, transverse at level of umbilical portion of left umbilical vein where it is equidistant from sides of abdo; ideally at confluence of L and R portal vv and where abdomen is round.
- Femur length (FL) measures ossified portion of femoral diaphysis, excluding distal femoral epiphysis (doesn’t fuse with shaft in-utero) and cartilages (distal femur point = artifactual thinner echogenic line of fibrous tissue around cartilaginous end of shaft only seen when perpendicular). Entire femur must be seen, be perpendicular to beam and centered so there is shadowing. Normally lateral edge is straight, medial edge is curved.
Composite age combining BPD, HC, AC and FL more accurate in early pregnancy (to 1/52 at 10-20/40, 2/52 at 20-30/40, 3/40 at 30-40/40), body parts with structural anomalies should be excluded. Age at 1st US is the most accurate, and is not changed thereafter.
Estimated foetal weight (EFW) taken from AC, BPD or AC and FL; is accurate to 17-20% when using 2 parameters, 15% for 3. Foetal growth is most rapid in the early 2nd T, reducing until term. Interval rate can be calculuted at average mm/week for the mean GA of the correspoding studies and compared with charts. Growth studies should be at least 4/52 apart due to measurement error.
Intrauterine Growth Retardation (IUGR)
(Foetal growth restriction, FGR). 5% of births. 4-8x risk of intrauterine demise, perinatal mortality. 50% of survivers have morbidity eg intrapartum foetal distress, hypoglycaemia, hypocalcaemia, meconium aspiration pneumonia, impaired immune function, retarded neurologic development, learning disabilities. Small for gestational age (SGA) if weight <10th centile (includes constitutionally small and IUGR). IUGR when EFW <6th percentile; excluded when above 20th percentile. If 6th-20th percentile then IUGR if there is oligohydramnios or maternal HTN; US sensitivity of 70%. If early US is not available GA taken from BPD, HC and FL (excluding AC). Growth consistently above 10th percentile or spurts from below to above the 10th is normal. Continued growth below 10th percentile or growth slowing (esp from above to below 10th percentile) is abnormal.
Intrinsic/foetal causes (chromosomal abnormality, intrauterine infection, structural abnormality/CHD, teratogen exposure) have fixed defects, don’t benefit from early delivery. Extrinsic causes usually benefit from therapy eg early delivery and include maternal causes (young age, HTN, chronic maternal disease eg anaemia or renal failure, preeclampsia, malnutrition esp prolonged hypoglycaemia, smoking, alcohol, drug abuse, inherited thrombophilias) and placental causes (infarction, abnormal cord insertion, placenta previa, abruption, primary placental insufficiency, premature placental aging/dysmaturity, vascular incl anticardiolipin antibodies and hypercoagulable states, placental tumours, multiple gestation, placental mosaicism). Placental mosaicism is common, mutation affecting a portion of the placenta. Persistently elevated α-FP suggests increased placental permeability, higher risk of later IUGR. Assymetric IUGR in up to 90% from malnutrition (esp placental or maternal causes) in late 2nd/3rd T, abdomen relatively smaller (reduced glycogen stores in foetal liver, reduced SC fat), then BPD then FL; HC:AC ratio >1 in 2nd T (>0.9 in 3rd T); commonly associated with oligohydramnios. Symmetric IUGR from reduced cellular growth usually starts in 1st T (eg chromosomal abnormality, drug toxicity, in utero infections) affects head, abdomen and femur equally; amniotic fluid usually normal.
IUGR followup every 1-2/52 with growth parameters, amniotic fluid volume, biophysical profile and UA Doppler. Normal weight gain in 3rd T is 100-200g/week. AFI <5cm poor prognosis.
Gestational age <37/40, 12% of deliveries. Increased risk woth PROM, intrauterine infection, uterine cervical and placental structural abnormalities (fibroids, cervical incompetence, placenta prevea, placental abruption), multiple gestation. Baby has increased risk of RDS, neonatal jaundice, NEC, spesis, GMH, developmental delay. Low birth weights are often appropriate adjusted for GA, worse prognosis than same weight in later GA due to organ immaturity.
Biophysical Profile (BPP)
Assessment of compromised/hypoxic foetus in late 2nd/3rd T. Linear association between foetal distress/demise and BPP. Acute hypoxia assessed with reactive foetal heart rate, respiratory activity, gross motor movements, foetal tone; each characteristic is from a different CNS centre. Chronic hypoxia is assessed with amniotic fluid volume. Each given score of 2 for normal, 0 for abnormal; total score 0-2 extreme risk of demise in 1/52, 8-10 no immediate risk. Normal foetus has sleep-wake cycles of 20-40min’ when asleep it cannot be spontaneously awakened and may resemble hypoxia, hence need to observer for 30min if abnormal BPP.
Placenta comprises ~100 maternal spiral aa (from uterine aa), 1 or 2 umbilical aa, long tortuous dilated capillaries in low-resistance/high-flow. Spectral Doppler of UA done in a free-floating loop of cord. Resistance in placenta estimated by S/D ratio or resistive index (RI = S-D / S). Tracing should have at least 5 consecutive comparable beats (without breathing, arrhythmia, gross foetal movement, hiccup) and have umbilical venous flow below baseline. Progressive loss of resistance with GA and trophoblastic invasion. High resistance (in placenta, vessels supplying placenta, foetus or maternal cardiovascular problems) if raised RI, S/D >4.0, early diastolic notch or absent forward diastolic flow (reversed flow high risk of demise in 1-7/7). This is less useful in foetuses <~20/40 (unable to deliver anyway), and less reliable in later 3rd trimester (hence MCA should also be performed).
MCA Doppler satisfactory tracing criteria as per UA. Normally diastolic flow is very low (high resistance) with RI >0.7. With abnormal UARI, MCA RI is initially normal/low (brain sparing) before increasing as cerebral autoregulation fails.
Ductus venosus tracing performed when MCA abnormal. Tracing abnormal if there is absent or reverse diastolic flow during atrial contraction, marked increased pulsatility.
EFW >90th percentile or EFW >4,000g. Large for gestational age (LGA) symmetrically large with evenly distributed fat. Macrosomia is asymmetrically enlargement with fat more in shoulders and body, higher risk of Cx. Increased risk in maternal diabetes, obesity, previous macrosomia, excessive weight gain during pregnancy. Complications include shoulder dystocia, traumatic delivery, fractures, brachial plexus injury (Erb’s palsy), perinatal asphyxia, neonatal hypoglycaemia, meconium aspiration.
Malformations are primary errors of morphogenesis. Disruptions are extrinsic disturbances in morphogenesis (eg infection). Deformations are extrinsic disturbances from abnormal biomechamic forces (eg uterine constraint, oligohydramnios, abnormal fetal position). A sequence is a cascade of anomalies triggered by an initiating abnormality (eg Potter sequence). A syndrome is a constellation of anomalies that are pathologically related with a common initiating defect (cf sequence). Agenesis is complete absence of an organ and its primordium; aplasia is absense due to failed developemnt of the primordium; atresia absent opening; hypoplasia incomplete development (reduced size and number of cells); hyperplasia is enlargement with too many cells; dysplasia (in malformations cf neoplasms) is abnormal organisation of cells.
Insults in the early embryonic period (first 3/52 after fertilization) leads to abortion or recovery without defects. Between 3-9/40 there is organogenesis with extreme suseptibility to teratogenesis (esp 4-5/40). The fetal period is susceptibile to growth retardation or injury to the already formed organs. Anomalies may be due to:
- Genetic – Associated with chromosomal abnormalities or single-gene mutations (mendelian disorder).
- Environmental causes – Infections, drugs (thalidomide, folate antagonists, androgens, alcohol, anticonvulsants, warfarin, retinoic acid), radiation, maternal diabetes.
- Cyclopamine (roots of the California lily) inhibits Hedgehog gene signalling, increasing hyloprosencephaly and cyclopia.
- Valproic acid (anti-epileptic) causes abnormal patterning of limbs, vertebrae, craniofacial structures.
- Excess Vitamin A (retinol, for treatment of acne) causes CNS, cardiac and craniofacial defects with cleft lip/palate.
- Multifactorial – From multiple susceptible genes and interaction with the environment.
Ranging from generalised oedema (hydrops fetalis, usually lethal) to more localised pleural/peritoneal effusions, cystic hygroma. Pathologic accumulation of fluid in body cavities and tissues with ascites, pleural and pericardial effusions (>2-7mm), subcutaneous oedema, placental enlargement, polyhydramnios. Skin thickening alone (± placental enlargement, polyhydramnios) is more likely gestational DM. Anaemia causes increased vascular velocities (MCA measured >70cm/s), extramedullary haematopoiesis with enlarged liver, spleen, LN. In failure RV fails 1st with tricuspid regurgitation, A wave (atrial contraction).
- Immune hydrops – Previously most common. Maternal exposure to foetal antigens, most commonly rhesus (Rh) D antigen incompatibility others include anti-kell, anti-ABO, anti-M. Foetal red cells reach maternal circulation during last trimester or during childbirth, with mother becoming sensitised. Usually in the second trimester IgG crosses placenta causing foetal haemolytic anaemia -> if severe there is hypoxic injury to heart and liver -> heart failure and reduced plasma protein synthesis -> oedema, jaundice causing kernicterus. Tx foetal transfusion, highly successful. Prevented by anti-A or anti-B IgM Rhesus antibody (RhIg) with anti-D antibodies that do not cross the placenta, preventing maternal immunisation; given at 28/40 and within 72hrs of delivery.
- Nonimmune hydrops – Infections present with combination of causes. Down syndrome rarely presents with hydrops. Prognosis poor.
- Cardiovascular – severe arrhythmia, structural anomaly, myocarditis, AV shunt eg foetal/placental tumours, twin-twin transfusion, high output failure
- Chromosomal abnormalities – Turner (generalised lymphatic obstruction esp cystic hygroma), trisomy 21 and 18.
- Foetal anaemia – Homozygous thalassaemia, parvovirus B19 infection (apoptosis of erythroid procursors)
- Reduced oncotic pressure – Hepatitis, nephrotic syndrome
- Increased capillary permeability – Anoxia
- Obstructed venous return – Mass eg CCAM, CDH
High risk with advanced maternal age >35yo, previous child with abnormality, foetal structural abnormality (11-35% esp holoprosencephaly, Dandy-Walker, cystic hygroma, cardiac, omphalocoele, duodenal atresia, facial, early symmetric IUGR). Confirmed with amniocentesis (unable to be done in early pregnancy, pregnancy loss rate 1:200-300) or chorionic villous sampling (CVS). Estimated up to 90% of chromosomal abnormalities can be detected by screening.
Triple marker screening at 16-18/40 includes β-hCG and unconjugated estriol (uE3) in conjugation with MS-AFP, reported as multiples of the median (MOM). α-fetoprotein (AFP) is produced by the amniotic sac then foetal liver, levels highest in foetal serum, small amount in amniotic fluid (AF-AFP), minute amounts in maternal serum (MS-AFP). AFP elevated when >2.50 MOM, referred for detailed US including dating ± amniocentesis. Levels correlated with maternal age to determine risk of eg trisomy 21/18. Low MS-AFP in Down syndrome. Raised MS-AFP in erraneous dating, multiple pregnancy, foetal demise, open neural tube defects (anencephaly, spina bifida, encephalocoeles), abdominal wall defects (gastroschisis, omphalocoele), amniotic band syndrome, cystic hygroma, placental abnormality (subchorionic haemorrhage, chorioangioma) or unexplained (higher risk of IUGR, foetal death, preterm delivery, preeclampsia, oligohydramnios).
In NZ maternal serum screening (MSS1 in 1st T, MSS2 in 2nd T) includes βhCG, pregnancy-associated plasma protein-A (PAPP-A) AFP, uE3 and inhibin A. If low risk then soft markers later in pregnancy can be ignored, but hard markers esp absent nasal bone and nuchal fold thickening remain high risk.
Nuchal translucency (NT) – Hypoechoic region posterior neck measured from CRL 45-84mm (~110-136/40) in true sagittal plane between inner margins of the lucency. Foetus must occupy most of the image, nasal bone visualised, neutral position (not flexed or extended). NT 2.0-2.5 risk varies with CRL and maternal age; >2.5 always increased risk. Risk of aneuploidy (esp trisomy 21, others including Turner syndrome, trisomy 13/18, triploidy), cardiac abnormality, diaphragmatic hernia, abdominal wall defect, skeletal dysplasia, other foetal syndromes, foetal demise. Even if MSS1 is low risk, NT >2.5mm should have detailed foetal echo at ~19/40. Thickening may resolve after 14/40, hence different etiology than nuchal thickening.
Soft markers – amniocentesis if 2 or more (in absence of MSS1):
- Maternal age
- Choroid plexus cyst – usually seen at 16-21/40, commonly regress. Unusual at 26/40.
- Renal dilatation/pyelectasis >4-5mm
- Sandal toe gap
- Single umbilical artery
- Echogenic cardiac focus – present in up to 20% of Asians.
- Absent/small stomach
- Mild ventriculomegaly 10-12mm – repeat scan at 26-32/40. Choroid separation >/=3mm from medial wall lateral ventricle.
- Short femur
- Clindodactyly – little finger angled inwards
- Nuchal fold thickening – Nuchal fold is soft tissue immediately posterior to occipital bone on transcerebellar plane measured midline AP at 16-22/40 in bregmatic plane (transverse, suboccipital). Thickened >/=6mm.
- Absent nasal bone – visible in 99.5% of chromosomally normal foetuses; absent in 70% of aneuploidy.
- Pericardial effusion >2mm – look for other cardiac anomalies, evidence of hydrops (could be earliest sign).
- Reversed A wave in ductus venosus, tricuspid regurgitation.
- Pleural effusion or ascites
- Echogenic bowel greater than bone with gain turned down until bone disappears. Harmonic imaging should be off. DDx CF, infection, swallowed blood.
- Persisting clenched hand
- Cardiac anomaly
- Congenital diaphragmatic hernia
- Spina bifidia
- Facial cleft esp midline
- Rocker bottom foot
- Duodenal atresia
- Trisomy 21 (Down syndrome) in 1/660 (1/250 >35yo, but 80% born to <35yo), 60% detected by triple marker serum screening where α-fetoprotien and unconjugated oestriol levels low, hCG high. Associations include CHD (esp AVSD), duodenal atresia, hydrocephalus, nuchal fold thickening, absent nasal bone, short femur/humerus, echogenic bowel, omphalocoele, renal pyelectasis, intracardiac echogenic focus, hypoplastic middle phalanx 5th finger, sandal gap. 50% have no US abnormality.
- Trisomy 18 (Edward syndrome) in 1/3,000. α-fetoprotien, unconjugated oestriol levels and hCG all low. Associations include IUGR (74%), complex CHD (52%), choroid plexus cyst (50%), Dandy-Walker, callosal agenesis, neural tube defects, facial clefts, congenital diaphragmatic hernia, omphalocoele, pyelectasis, horseshoe kidney, clenched hands with overlapping digits, radial ray abnormal, clubfeet, rockerbottom deformity, single umbilical a, ‘strawberry sign’ (flattening of occiput from hyoplastic hindbrain, flattened frontal bones from hypoplastic frontal lobes).
- Trisomy 13 (Patau syndrome) in 1/6,000, few liver more than a few days old. Associations include CHD, holoprosencephaly, ventriculomegaly, callosal agenesis, enlarged cysterna magna, neural tube defects, facial clefts, polydactyly, rockerbottom foot, clenched hands with overlapping digits, bladder exstrophy, omphalocoele. DDx Meckel Gruber syndomre with cystic kidneys (enlarged, hyperechoic), abnormal head (encephalocoele or microcephaly), polydactyly.
- Turner syndrome (45XO) in 1/2,000-5,000. Cystic hygroma (septated) with thickened nuchal fold/translucency, aortic coarctation, horseshoe/pelvic kidney, mild IUGR, short limbs, hydrops fetalis (lymphatic failure).
Transcervical/ascending infections include most bacteria and some viruses (eg HSV2). The foetus becomes infected by inhaling infected amniotic fluid into the lungs shortly before birth, or during passage through the infected birth canal. Infection may damage and rupture the amniotic sac, or cause inflammation, inducing labour. Usually causes pneumonia, sepsis and/or meningitis.
Transplacental/haematologic infections occur at any time during gestation, occasionally at time of delivery (hepatitis B, HIV). TORCH group of infections have similar manifestations including encephalitis, chorioretinitis, hepatosplenomegaly, pneumonitis, myocarditis, haemolytic anaemia, skin lesions:
- Toxoplasma gondii (2nd most common) – scattered brain calcifications (esp basal ganglia, most common infection to calcify, resolves with Tx), hydrocephalus (synechiae at aqueduct or foramina of monro/Magendie/Luschka, hydranencephaly (if severe), multicystic encephalopathy, porencephaly.
- Others – incl syphilis causing seizures, CN palsies. Optic atrophy, tabes dorsalis, meningitis, vasculitis/infarctions, enhancing meninges and perivascular spaces.
- Rubella – At risk period from just before conception to 16/40 esp 1st 8/40. Cataracts, chorioretinitis, glaucoma, heart defects (PDA, PA hypoplasia/stenosis, VSD, TOF), deafness from sensorineural injury. Uncommon brain injury (microcephaly, vasculopathy, massive calc).
- Cytomegalovirus (CMV, most common) – At risk period 2nd trimester, usually haematogenous maternal spread. May cause necrotising periventricular infection (periventricular calc, subependymal cysts esp around occipital horns, atrophy/microcephaly), migrational abnormal (cortical dysplasia), cerebellar hypoplasia, ventricular enlargement/porencephaly. Hearing impairment, mental retardation, developmental delay.
- Herpes simplex type 2 – From delivery in mothers with genital herpes. Similar to CMV with cystic periventricular encephalomalacia (haemorrhagic infarction, scattered brain calcifications); but microcephaly and micophtalmia more common, retinal dysplasia.
- HIV – primary HIV involvement (progressive encephalopathy, diffuse atrophy, delayed myelination, basal ganglia and frontal lobe subcortical white matter calcifications) or secondary complications (infection, tumour, vasculitis/aneurysms). 90% of HIV infected infants get AIDS in 1st 2yrs, and may develop AIDS encephalitis, infections, lymphoma. PML, toxoplasmosis and TB rare in children.
Absent cardiac activity, absent foetal movements, overlapping of skull bones (spalding sign), gross distortion of anatomy (maceration), soft tissue oedema (skin >5mm), thrombus of gas in foetal heart.
Risk of CNS abnormality is 0.1%, increasd to 2% with previous child CNS anomaly. If all 3 planes are normal, risk of CN anomaly -> 0.005%.
- Transthalamic plane – Includes thalami (hypoechoic) or midbrain, third ventricle (single echogenic line or slitline <3.5mm), cavum septum pellucidum. Cerebellum should not be included. For BPD, HC, head shape, microcephaly, macrocephaly, major structural abnormality.
- Transventricular plane – axial at ventricular atria including echogenic choroid plexus (normally fills 60-90% ofthe atrium). Measure atrial diameter perpendicular to walls (normal </=10mm). Both lateral ventricles should be measured. Near-field may be limited by reverberation, improved by turning/tilting transducer to image via lambdoid/coronal sutures.
- Transcerebellar plane – axial with 10-15deg inclination from cantomeatal line, includes inferior 3rd ventricle, thalami, upper cerebellar hemispheres, cisterna magna. Cerebellum is bilobed hypoechoic with smaller midline hyperechoic vermis, measured axially from outer edges. Cisterna magna normally 2-10mm width from vermis to inner occiput; <2mm Chiari II, massive ventriculomegaly; >10mm mega cisterna magna, Dandy-Walker, arachnoid cyst, cerebellar hypoplasia. Cisterna magna may have arachnoid septa. 4th ventricle enlarges with gestation, up to 8mm at term, may be slit-like, triangular, oval or ‘boomerang’-shaped.
From increased intraventricular pressure (obstructive hydrocephalus, associated with head enlargement), cerebral atrophy or maldevelopment (eg agenesis of the corpus callosum). Most commonly Chiari II and aqueduct stenosis. 80% associated with other abnormality. Increased risk of aneuploidy. Ventricular atrium >10mm, separation of choroid plexus from ventricular wall >3mm, dangling/dependent choroid which is reduced in size. 3rd ventricle >3mm, rounded/triangular. Disrupted falx midline echo and residual rim/mantle of brain parenchyma (cf hydranencephaly).
Significant when BPD or HC < 3 SD below the mean, or lags behind body by >/= 3/40. From infection (rubella, CMV, herpes simplex), developmental (esp holoprosencephaly), trisomies, pollutants (radiation, toxins, heavy metals). If associated with ventriculomegaly it is usually atrophy of brain parenchyma causing ex-vacuo effect. Intracranial calcifications (may not have shadowing) suggests CMV (in normal-size head suggests toxoplasmosis), DDx tubers in TS.
Chroid plexus is easily compressible eg in hydrocephalus. Rich source of glycogen, if small in absence of hydrocephalus suggests impaired brain development.
Choroid plexus cysts in 1-3% of normal foetuses in 2nd T, circular, nearly always resolve. Present in 50% of patients with trisomy 18, hence detailed US (heart, face, hands) should be done, amniocentesis only if there are other abnormalities.
Most common neural tube defect, inevitably fatal. Absent cranial vault and cerebral hemispheres above level of orbits creating ‘frog-like’ appearance. Space may be replaced with amorphous neurovascular mass = area cerebrovasculosa or angiomatous stroma. Polyhydramnios common.
‘Water head’, lethal. Total destruction of cerebral cortex ?from occluded ICAs with ventricular dilatation to fill the void. Occipital lobes, brainstem, posterior fossa usually spared as they are supplied from intact posterior circulation. May have iregular areas if destruction is ongoing, or complete absence of brain if complete. No cortical mantle of brain tissue. Head usually enlarged ?impaired CSF resorption. Falx midline echo usually maintained, but may be disrupted due to raised intraventricular pressure.
Failure of prosencephalon to divide and form separate R/L hemispheres, thalami. Associated with trisomy 13, 18P syndrome, 13Q syndrome, triploidy. Associated with midline abnormalities including hypotelorism, cyclopia, proboscis, midline cleft, heart and abdominal anomalies. Head size may be normal, macrocephalic or microcephalic.
- Alobar holoprosencephaly – absent falx and interhemispheric fissure, single midline ventricle (may communicate with dorsal cyst), fused thalami and basal ganglia, absent cavum, facial abnormality. Surrounding anterior wedge of parenchyma resembles a boomerang.
- Semilobar holoprosencephaly – monoventricle anteriorly communicating with partially formed lateral ventricles posterioly, fused thalami and basal ganglia.
- Lobar holoprosencephaly – Absent frontal horns of lateral ventricles, absent cavum septum pellucidum, normal head size. DDx agenesis of corpus callosum.
Fluid (meningocoele) or brain (encephalocoele) filled sacs protruding through calvarial bony defect in occipital (75%), frontoethmoid (13%) or parietal (12%) regions. Reduced BPD and HC. Highly associated with CNS and extra-CNS anomalies, aneuploidy.
Chiari II Malformation
Caudal displacement of cerebellar tonsils, pons and medulla, elongated 4th ventricle, small posterior fossa, obliterated cisterna magna. Almost always caused by and associated with an open neural tube defect (ONTD), occasionally closed. Small/obliterated cisterna magna (<2mm). Reduced size and distorted shape of posterior fossa, banana sign – compression of cerebellar hemispheres into banana shape curved anteriorly. Altered head shape, lemon sign – flattening/concavity of bilateral frontal bones with bossing anteriorly (also may be normal, seen with other structural abnormalities eg omphalocoele). Hydrocephalus common, from obstructed 4th ventricle outflow.
(Cystic dilatation of the 4th ventricle). Maldeveloped roof of 4th ventricle. Enlarged cisterna magna communicating with 4th ventricle through the defect, enlarged posterior fossa, elevated tentorium. Cerebellar hemispheres usually hypoplastic, usually hydrocephalus. Dandy-Walker variants less severe with normal sized posterior fossa, mild vermian hypoplasia, less ventriculomegaly. Associated with midline anomalies including agenesis of corpus callosum, cephalocoele, meningocoele, cardiac, genitourinary, GI, MSK, aneuploidy. DDx arachnoid cysts (rounded margins cf triangular Dandy-Walker cyst), mega cisterna magna (lack of communication with 4th ventricle), vein of Galen aneurysm. NB cerebellum normally not completely formed until 18/40, before this there may be a normal small vermian cleft. Cisterna magna may be falsely enlarged by too steep angulation towards coronal with visualisation of cervical spine on same image, rounded foetal head.
Vein of Galen Aneurysm
Misnomer, marked dilatation of vein of Galen due to communication between carotid/vertebrobasilar and venous plexus. Rounded/oval fluid in midline postero-superior to 3rd ventricle/thalamus, tubular in sagittal extending to sagittal sinus, high velocity tubulent Colour flow. May cause high-output cardiac failure, hence need to check for cardiomegaly, hydrops. May obstruct aqueduct causing hydrocephalus.
Agenesis of the Corpus Callosum
May be isolated or associated with chromosomal abnormalities, CNS abnormality (Dandy-Walker, holoprosencephaly, Chiari II) and other syndromes. Antenatal diagnosis is dificult, but may see ventriculomegaly with lateral ventricles more lateral and parallel (racing car sign), small frontal horns, colpocephaly (tear-drop enlargement of posterior horns), 3rd ventricle enlarged and more cephalad which may herniate between hemispheres (midline interhemispheric cyst), absent cavum septum pellucidum, gyri radiate from interhemispheric fissure.
Echogenic without shadowing in region of germinal matrix or lateral ventricles. DDx enlarged choroid plexus, intracranial tumour eg teratoma.
Parenchymal loss with cystic areas in a vascular distribution (?infarct), often communicating with ventricle, ex-vacuo ventriculomegaly. May enlarge with gestation due to impaired CSF resorption. May cause mass effect, herniate through cranial suture.
Face, Neck and Spine
Hyperechoic ossification centres of spine in anterior centrum (body), two posterior elements (laminae). Hence a ‘sagittal view’ is actually oblique showing 2 of these. Spinal canal should taper caudally. L5 ossifies by 16/40 with each sacral level ossifying every 2-3 subsequent weeks.
Neural Tube Defects
Anywhere in spine, most common lumbosacral. Outward splaying (cf inward convergence) of laminae, overlying soft tissue defect, protruding sac with fluid ± thin strands (may be nerve roots or arachnoid bands). May be associated with intraspinal lipoma or diastematomyelia. If severe, may have sharp angulation/deformity of spine at that level. Associated with club foot or dislocated hips (due to neuromuscular defect), Chiari II (in 95%), ventriculomegaly (in 75%), aneuploidy (in 4%, esp trisomy 13/18).
- Spina bifida occulta – nonfusion of vertebral arches with intact skin.
- Meningocoele – only contain meninges, CSF.
- Myelomeningocoele (meningomyelocole) – protruding sac containing CSF and neural elements (spinal cord or nerve roots).
- Myeloschisis – totally open spinal defect.
Variable solid cystic mass adjacent to sacrum and coccys. Type 1 completely external, type 4 completely internal. Can be very deforming. Malignant degeneration uncommon in utero, may occur if left. If high-flow may cause foetal comprimise. If very large then C-Section may be required. Teratomas also seen in face, neck, intracranially.
- Lateral/eccentric cleft (most common) affect both lip and palate (hard ± soft) in 50%, lip alone in 25%, palate alone in 25%, bilateral in 25%. 60% have other anomaly eg polydactyly, CHD, trisomy 21. Groove extending from nostril through lip best seen on coronal lips and nose view. Palate involvement best seen on axial as break in C-shaped maxilla.
- Medial/central cleft (uncommon) associated with holoprosencephly, aneuploidy. Wide central defect upper lip and palate.
- Interorbital distance measured on axial or coronal as outer-to-outer diameter (OOD) of bony orbits. Reduced in hypotelorism, increased in hypertelorism.
- Micrognathia (small chin) associated with trisomy 13/18, triploidy. True midline sagittal mandible whoing receding chin, may be associated withpolyhydramnios if swallowing is impaired.
- Facial tumours include haemangiomas (hyperechoic, compressible), teratomas (solid ± cystic, may have polyhydramnios from impaired swallowing).
Failed lymphatic connections with veins in neck. Uni/bi-lateral nuchal cystic mass, prominent midline septum (nuchal ligament) ± septations in spoked-wheel pattern. May be isolated or associated with diffuse hydropic lymphatic obstruction with skin thickening and body cavity fluids (always fatal). 70% have chromosomal abnormality (Turners, trisomy 21/18). Cystic hygromas arise in the jugular lymphatic region; lymphocoeles can be in any other areas of the body, not associated with chromosomal abnormalities or diffuse obstruction.
Chest and Heart
Heart motion assesmment includes rate, rhythm (constant or variable), atrial to ventricular association, structural abnormalities and evidence of foetal hydrops. Foetal heart rate bradycardic when <90bpm at 5-6/40, or <120bpm >8/40. Transient bradycardia can be normal as long as it isn’t <60bpm or >15-20sec. There is no established upper limit of concern. M-mode if abnormal, ideally through ventricle, AV valve and atrium.
- Four chamber view – axial just above diaphragm. Apex points to the left at ~45deg on same side as stomach. Ventricles approximately equal size, slightly smaller than atria. Papillary muscles may be echogenic and prominent. Discrepancies in chamber size or valve motion suggests malformation. Normal heart occupies ~1/3 area of thorax, should not exceed 50%. RV is most anterior, LA closest to descending aorta and spine. Valvular offset – tricuspid valve projects further into RV than bicuspid into LV. Medial leaflet of tricuspid valve opens hard against interventricular septum (cf bicuspid limited by LVOT).
- 3-vessel/base view – axial just cephalad to 4ch view. Med -> lat: PA, aorta, SVC (PAC); posterior trachea. PA:Ao 1:1. RVOT crosses LVOT. May see PA bifurcation. Large aorta in TOF, trunchus arteriosis. Small aorta in coarctation, hypoplastic left heart. Small PA in hypoplastic right heart, Ebstein anomaly with pulmonary hypoplasia.
- Long LVOT – LV with transducer angled to right shoulder. Continuity of LV with ascending aorta. More complete visualisation of ventricular septum.
- Long RVOT – RV with transducer angled to left shoudler.
- Additional views:
- Aortic arch
- Ductal arch – curve not as tight as LVOT.
- Pulmonary veins
- SVC -> RA
Gernerally flow through any valve has maximum normal velocity 100cm/s.
Pericardial effusion – anechoic band >2mm, 2-7mm may be OK if isolated. Pleural effusions may be associated with hydrops, extracardiac mass or chylothorax (lymphatic obstruction L>R; if large may need thoracentesis just before delivery).
Distal airways and terminal air sacs continue to develop after birth. Any compression (mass, small chest, oligohydramnios) inhibits this and causes pulmonary hypoplasia. Normal thoracic:abdominal circumference (at level of 4ch) is >0.75.
Foetal Cardiac Anomalies
- Ventricular hypoplasia – smaller chamber with apex shortened.
- Axis rotation away from 45deg is associated with other cardiac anomalies.
- Septal defect – ventricular, atrioventricular (endocardial cushion) or atrial septal defects. DDx membranous septum near mitral/tricuspid valves (hypoechoic or appear absent with parallel beam, reappears with adjusted scan plane) or foramen ovale.
- Rhabdomyoma – hyperechoic from muscle walls or septa, may compress chambers. Associated with tuberous sclerosis.
- Papillary muscle calcification – small hyperechoic foci, LV>RV, usually no shadowing. Not associated with other structural anomalies, but 2x increased risk of trisomy 21.
Congenital Diaphragmatic Hernia
Sporadic, 1:2,000. Highly associated with cardiac, CNS defects, aneuploidy. Abdominal contents protrude through diaphragmatic defect. Bochdalek (90%) at posterolateral aspect, 75% left-sided. Morgagni (10%) antero-medially. Fluid-filled, solid or multicystic mass in chest, displaced heart/mediastinum, absent stomach in abdomen, polyhydramnios (if obstructed). 50-80% mortality from pulmonary hypoplasia, may be bilateral due to mediastinal shift. DDx large diaphragmatic eventeration with diaphragm elevated due to muscular hypoplasia/aplasia where the ipsilateral lung may be compressed but contralateral lung normal.
Congenital Cystic Adenomatoid Malformation (CCAM)
Lung hamartoma usually of one lobe. Type 1 (most common) single/multiple cysts >20mm; type 2 multiple small cysts uniform size <20mm, type 3 echogenic solid masses from microscopic cysts <5mm. Associated with polyhydrambnios, fotal hydrops. Not associated with other anomalies or aneuploidy. Some resolve spontaneously in utero. DDx temporarily obstructed bronchus from mucus plug.
Mass of lung tissue supplied from systemic aa, separated from bronchial and pulmonary vascularity. Most posterior basal segments lower lobe, L>R. Intralobar (75-85%) within pleural covering, pulmonary venous drainage maintanied, rarely detected in utero (?acquired). Extralobar (15-25%) are accesssory lobes within own pleura, supplied from both systemic aa and vv. Homogeneous, echogenic, solid mass ± cystic displacing mediastinum; extralobar typically conical/triangular associated with hydrops or pleural effusion.
Rare obstruction of tracheobronchial tree (from larynx to bronchi). Lung enlarged, echogenic from retained secretions, compresses heart, inversion of hemidiaphragm.
Normally relatively large cf body length. Pelvis relatively small.
Liver is relatively large (glycogen store and haematopoiesis), R lobe measured from dome to caudal tip. Enlarged with hydrops (esp immune), infection, diabetes. Small in IUGR. Cysts are typically anechoic; haemangiomas hyperechoic. Calcifications may be from infection (toxoplasmosis, CMV, herpes, varicella), tumours, peripheral emboli/infarction, idiopathic (single); DDx perihepatic calcification of meconium peritonitis. Pseudoascities – hypoechoic curved line between liver and anterior abdominal wall representing hypoechoic muscles, doesn’t extend beyond ribs. Gallblader is typically teardrop shaped.
Adrenals 20x relative size (cf adults), 1/2 size of kidneys as thin disc-like paraspinal mass with hypoechoic cortex and hyperechoic medulla. In renal agenesis, may take on shape of kidney in renal bed. Rare cases of in-utero adrenal haemorrhage and neuroblastoma.
Ascites – Visualisation of outer echogenic margin of intraabdominal umbilical vein, falciform ligmant, surrounds abdominal organs.
Varix – dilated extrahepatic umbilical vein, occasionally associated with foetal hydrops, structural anomaly, aneuploidy, early 3rd T demise.
Cystic masses include ovarian cyst, hydrometrocolpos (obstructed uterus and vagina), mesenteric or duplication cyst, urachal cyst, choledochal cyst. Complex and solid masses are rare; may represent meconium pseudocyst, renal or adrenal mass.
Failure to see stomach by 18/40 is abnormal in 1/2 (rest normal variant or recent gastric emptying), but should always be seen on 60min of scanning. From obstruction (oesophageal atresia, chest mass), impaired swallowing (facial clefts, neuromuscular disorder), low amniotic fluid, ectopic stomach (diaphragmatic hernia); follow-up warranted. Gastric pseudomass – conglomerate swallowed echogenic debris within stomach, most normal but some associated with slowed bowel transit (obstruciton), blood in amniotic fluid (abruption, amniocentesis).
High GI obstruction (proximal to ligament of Trietz)
- Oesophageal atresia may be associated with treacheooesophageal fistula (high incidence of other anomalies).
- Gastric obstruction rare.
- Duodenal obstruction causes double-bubble sign (fluid distension of stomach and proximal duodenum) with polyhydramnios. From duodenal atresia (associated with Downs), webs, stenosis, annular pancreas, volvulus/malrotation, bands.
SB is moderately echogenic, central, blends with liver, peristalsis seen by 3rd T, normal <6mm diameter. Colon seen by 20/40, may see haustra, progressively fills with meconium (echogenic in 2nd T, becoming progressively hypoechoic by term, but is variable). Bowel obstruction may not be apparent in-utero, DDx dilated tortuous ureter. SBO (SB >7mm, LB >20mm may see 3 ‘cyts’ of stomach duodenum and upper jejunum, polyhydramnios common) from jejunal/ileal atresia/stenosis, enteric duplication. LBO (if low from volvulus may cause meconium calcification, polyhydramnios common ?cause) from meconium ileus (thick echogenic/heterogeneous meconium in distal ileum, almost always CF), anorectal stenosis/atresia (eg imperforate anus; 75% associated with other anomaly esp GU, VACTERL, caudal regression), Hirschsprung disease.
Meconium peritonitis from perforated bowel segment spilling meconium -> sterile peritonitis. 50% is idiopathic (?vascular insult to SB with spontaneous healing), rest from meconium ileus, bowel atresia, volvulus. Intraperitoneal calcifications in 85% (punctate/linear/clumpy, esp around liver, may extend into scrotum via patent processus vaginalis), meconium pseudocysts (fluid-debris level), ascities (w internal echoes), bowel dilatation, polyhydramnios.
Echogenic bowel likely involves SB and mesentery. Normal in 1/2, may be due to bowel wall interphases, echogenic contents, high frequency probe. Abnormal if echogenicity >/= bone, mass effect, isn’t transient, seen in different scan planes with different frequncy transducers; associated with CF, trisomy 21/18, SB atresia, volvulus, foetal viral infection (CMV), growth retardation, perinatal death. May be transient from swallowed blood in amniotic fluid (abruption, amniocentesis).
Anterior Abdominal Wall
All have skin surface disruption and hence elevated AFP.
- Omphalocoele (most common) – Abdominal contents -> base of umbilicus. Cannot be diagnosed <10/40 esp if <10mm due to physiological herniation of the gut. Covering membrane of peritoneum with cord inserting through. Large defects may contain liver and bowel, stomach, heart (ectopic cordis); from failed primary abdominal wall closure, associated with reduced AC. Smaller defects usually only contain bowel, may just be a ‘cyst’ of Wharton’s jelly at cord insertion; likely persistent primitive body stalk. 67-88% associated with anomalies including cardiac, CNS, urinary tract, GI malformations, trisomies (in 40%), Beckwith-Wiedemann syndrome (in 5-10%; gigantism, renal tumours, hemihypertorphy, macroglossia). Hernia may contain the heart (ectopia cordis). Ascites common, polyhydramnios in 1/3. DDx umbilical hernia which rarely occurs in-utero. Pseudo-omphalocoele – bulge from pressure on foetal abdomen by transducer, foetal limb, fibroid.
- Gastroschisis – Defect in all layers of anterior abdominal wall ~10-50mm (?infarct), typically RLQ. Bowel hierniates through (SB then LB, stomach, GU, shouldn’t include liver), free-floating in fluid without covering membrane. Small defects may -> bowel ischaemia -> thickening of bowel wall and dilatation (but this is also common without ischaemia). Cord insertion normal, lying next to the defect. Polyhydramnios uncommon unless obstructed. Most are not associated with chromosomal anomaly or recurrence. Increased risk with marijuana smoking. Tx postnatal repair, excellent prognosis.
- Pentalogy of Cantrell – Thoracoabdominal defect with ectopia cordis, omphalocoele, diaphragmatic defect, pericardial defect, disrupted sternum. Common cardiac anomalies. Poor prognosis.
- Limb-body wall complex – Neural tube defect, anterior abdominal wall defect, limb anomalies. Severe scoliosis common. Poor prognosis.
Kidney length ~GA in weeks, size <1/3 of abdomen, foetal lobulation, renal sinuses echogenic stripe. Kidneys normally start producing urine at 10/40, creating most of the amniotic fluid by 16/40. Ureters, if seen are dilated. Bladder should seen by 16/40 between iliac arteries on colour Doppler, cyclically (60-90min) filling and empty. If bladder <10mm or not seen after 1hr (fluid may be only mucosal secretions or residual urine from previously fuctional kidneys), renal function is impaired; from bilateral upper tract/renal abnormality (renal agenesis/hypoplasia, ARPCKD, bilateral MCDK), or generalised process eg growth restriction. Megacystitis is subjective, no established upper size limit. Normal amniotic fluid volume implies at least one functioning kidney.
Extrarenal pelvis if anechoic space is medial to kidney; pyelectasis if it extends into renal sinus. 40% have detectable renal pelvis distension (1-2mm), 20% large pelvis (>/= 3mm), likely from physiological VUR (N in 2nd and 3rd T), as long as ureters and calyces are not dilate). Hydronephrosis (from obstructive uropathy) if AP pelvis >5mm before 20/40 or >10mm after 20/40; or >50% AP kidney; or unequivocal caliectasis. Need to assess bladder filling and AFI. Dilatation in 2nd T should be followed at 30-34/40 (stop if <10mm). If <15mm postnatal AB prophylaxis, MCU and US at 6-8/52 (earlier false negatives from neonatal dehydration). If >/=15mm then admit and early US. If duplex kidney with no dilatation, US only at 6-8/52.
Obstructive uropathy may cause oligohydramnios if bilateral, can be treated with percutaneous catheters into amniotic cavity unless kidneys are dysplasic. Renal dysplasia is irreversible damage with cortical echogenicity > liver, cortical or subcortical cysts of varying sizes. Installation of saline may aid scanning or direct foetal urine analysis. Dysplasia later causes atrophy.
- Pelvicoureteric junction (PUJ/UPJ) obstruction – Pelvocaliectasis in ‘glove’ appearance (communicating cf MCDK). Ureter below PUJ normal and not seen.
- Duplex kidney – Lower moiety dilatation typically from reflux, upper pole from ectopic ureter (may see ureterocoele). Rarely bilateral, hence amniotic fluid volume normal (unless ureterocoele obstructes bladder outflow).
- Vesicoureteric (VUJ/UVJ) obstruciton
- Bladder outflow obstruction – From posterior urethral valves (males, occasional keyhole bladder with dilated proximal urethra) or less commonly cloacal/urethral atresia (females) causing early bilateral obstruction. In early gestation there may be only mild pyelectasis ?due to ‘rigid’ renal parenchyma, before dilatation. Chronic -> hypertrophied bladder (may become contracted). May have urinomas adjacent to kidneys.
Renal cystic disease:
- Cysts not normal in utero. Even solitary cysts may be associated with renal dysplasia, but may not afect renal function until postnatal.
- Multicystic dysplastic kidney (MCDK) – Multiple noncommunicating cysts of varying size (cluster of grapes), nonfunctional with no normal kidney identified, may have intervening echogenic tissue. May enlarge or plateau. Ureter atrophied. If unilateral then normal amniotic fluid volume, lungs mature normally. 20% have contralateral renal abnormality (VUJ/PUJ obstruction, reflux, subtle cysts, MCDK). Bilateral MCDK causes lethal oligohydramnios/pulmonary hypoplasia.
- Autosomal recessive polycystic kidney disease (ARPCKD, infantile PCK) – Presents later in pregnancy with massive enlargement of both kidneys, decreasing function and oligohydramnios, echogenic ± hypoechoic rim, discrete ‘cysts’ (tubular ectasia) not usually evident. Extension into paraspinal regions suggests a retroperitoneal lesion (adrenal or renal) cf other abdominal masses. Most die perinatally from pulmonary hypoplasia.
- Autosomal dominant polycystic kidneys disease (ADPCKD, adult PCK) – Rarely detected in-utero with enlarged kidneys, lack of sonolucent rim, occasionally discrete cysts.
- Meckel-Gruber syndrome – Cystic kidneys (enlarged, hyperechoic), abnormal head (encephalocoele or microcephaly), polydactyly. DDx trisomy 13 with enlarged hyperechoic kidneys, polydactyly, CNS anomalies.
- Cystic renal dysplasia from obstructive uropathy.
- Prune belly syndromes:
- Secondary – Prolonged overdistended bladder (occasional ascites or other mass) enlarging abdomen and causing thinned and flaccid anterior abdo wall muscles
- Primary (Eagle-Barrett syndrome) – Rare group of diseases in males causing abnormal mesenchymal development of ureters, bladder, abdominal wall with laxity, atypical obstructive uropathy, cryptorchism.
- Megacystitis microcolon hyperperistalsis intestinalis syndrome (MMIHS) – Rare, females. Polyhydramnios, enlarged bladder (much more than hydronephrosis), commonly enlarged ureters.
Vaginal atresia obstructs the vagina causing hydrocolops; if uterus dilated it becomes teardrop/ovoid shaped in hydrometrocolpos. Male scrotum or penis visible by 16/40, may see hydrocoele (typically resolves spontaneously after birth). Penis typically points cranially, clitorus caudally.
Caudal regression syndrome – Renal agenesis/dysplasia, sacral agenesis, lower limb hypoplasia, sirenomelia (rare).
Humerus:forearm and femur:leg ~1:1. Ulnar longer at the elbow, tibia longer and wider at the knee (cf radius, fibula). Diffuse skin thickening >10mm in fetal hydrops, maternal diabetes; focal thickening in mass (haemangioma, lymphangioma).
Normal foetuses intermittently open and close their hands and extend fingers; fixed clenched fist esp with overlapping index fingers suggests trisomy 18/13. Polydactyly is usually ulnar. Syndactyly is skin or bony fusion of digits eg trident deformity 2nd/3rd and 4th/5th fingers closely aligned. Clinodactyly is sharp deviation of fingers, Ectrodactyly (lobster-claw deformity). Hypoplasia middle phalanx 5th digit with femur and humerous shortening suggests Down syndrome. Clubhands and clubfeet are sharply flexed fixed deformity with sole in same plane as tib/fib; may be associated with skeletal dysplasias or MSK/neurological conditions (arthrogryposis).
Arthrogryposis multiplex congenita – Congenital joint contractures involving >/2 body regions. Abnormal limb positioning with clenched hands, knotted fingers, persistently extended/bent legs, clubbed feet. From syndromes, CNS anomalies (callosal agenesis, lissencephaly, hydrocephalus, aplastic vermis), neuromuascular disorders, severe oligohydramnios limiting movement.
Bilateral elongated FL is normal; unilateral suggests unilateral hypertrophy in Beckwith-Wiedemann syndrome. Bilatearl shortening is constitutional/normal (mild, appropriate interval growth), IUGR or skeletal dysplasia. Femur:foot length <0.9 suggests dysplasia, >0.9 usually constitutionally small or growth-retarded foetus.
Skeletal dysplasias – Abnormal size and shape of bones. May have shortened limbs, bony fractures, bowing (only medial femur is normally curved, other boes should be straight), reduced bone echogenicity (demineralisation), polydactyly, abnormal head shape, spine anomalies, midface hypoplasia, ventriculomegaly, polyhydramnios, hydrops, small thorax. Pulmonary hypoplasia predicted by lower thoracic:abdominal circumference ratio. Limb shortening may be micromelic (proximal and distal), rhizomelic (only proximal), mesomelic (only forearm/lower leg), or acromelic (only distal segments). Lethality predicted by micromelia, marked demineralisation, very small thorax.
- Thanatophoric dwarfism (28%) – Sporadic, lethal (from pulmonary hypoplasia). Severe micromelic limb shortening and deformity, small thorax with markedly straighted ribs, cloverleaf skull (kleeblattschadel deformity from craniosynostosis of sutures), large head, hydrocephalus, polyhydramnios.
- Achondroplasia (15%) – Autosomal dominant (lethal if homozygous), one parent must have the condition. Proximal limb shortening (rhizomelic) with short FL, may not occur until late 2nd T.
- Osteogenesis imperfecta (OI, 14%) – Group of disorders, most being autosomal dominant, 50% lethal. Demineralisation (in more severe forms), bone thickening with fractures and callus, bowing, small chest, protuberant abdomen. May not be affected in-utero.
- Achondrogenesis (9%) – Lethal, autosomal recessive. Severe proximal and distal limb shortening (micromelic) with deformity, short trunk, large head, distended abdomen, narrowed bell-shaped chest with short ribs.