First trimester is from conception to end of 13/40. Embryonic period is 0-10/40.
- Serum β-human chorionic gonadotropin (β-hCG) is positive within 1/52 of conception (~3/40 gestation). Normally doubles every 2 days. Usually peaks at 8-11/40 before declining.
- Decidual reaction – Hyperechoic endometrial thickening >8mm (less in ectopics). Poor prognosis if <2mm.
- Intradecidual sign 4/40 – Tiny cystic structure within echogenic decidua, eccentric to uterine cavity (specific), should be in upper uterine body; DDx fluid collection or decidual cyst. Low lying may be normal (more common in retroverted uterus), miscarriage, cervical ectopic or displaced by fibroids.
- Gestational (chorionic) sac by 5/40 – Smoothly contoured, round/oval, anechoic within endometrial cavity near fundus. Echogenic border/decidua >2mm (choriodecidual reaction). Usually seen when βhCG >1,000IU, if >2,000 and not seen suggests ectopic. Mean sac diameter (MSD, average of 3 planes measured from inner edge of hyperechoic rim) growth >1.2mm/day (10mm/week), is ~2mm at 5/40. GA in days ~ MSD + 25. Poor prognosis if abnormally large or small for GA.
- Double decidual sac sign (in 85%) – Visualisation of the three layers of decidual reaction (hyperechoic trophobastic tissue): decidua vera lining endometrial cavity, decidua capsularis covering gestational sac and decidua basalis forming placenta. Small amount of fluid in endometrial cavity separates the vera from capsularis.
- Yolk sac 5.5/40 – 2-6mm, spherical connected to midgut via thin vitelline/omphalomesenteric duct, floating freely between amnion and chorion. First structure seen in the gestational sac. Always seen in normal pregnancy when MSD >/=20mm, βhCG >7,200IU. Poor prognosis if >6mm, irregular shape, thick/echogenic/calfified wall.
- Embryo visible 6/40. Double bleb sign (uncommon) – Amniotic sac and yolk sac with embryonic disc between. Embryos as small as 2mm seen TV, always seen when GSD >/=25mm, βhCG >21,000IU.
- Cardiac activity should always been seen when CRL >/= 7mm. Rate initially >100bpm, increasing with gestation. <80bpm poor prognosis.
Gestational age estimated by MSD or CRL.
The chorion lines the inner surface of hyperechoic gestational sac, the amnion expanding to fuse with it at up to 16/40. Primary/primitive yolk sac develops and resolves by 2/52, not seen on USS. The secondary yolk sac develops within the chorionic space on a stalk before detaches from the embryo, begins to involute at 11/52. Poor prognosis if yolk sac >6mm or calcified.
Fluid in the uterine cavity may reflect gestational sac, normal chorioamniotic separation, ‘vanishing twin’ (failed twin, separate from normal gestational sac, distorted contour, almost always resolves by 2nd T), implantation bleed, or necrotic fibroid.
PV bleeding in the 1st trimester from:
- Implantation bleeding (subchorionic/perigestational/intrauterine haematoma) – Bleed in uterus where chorion inserts into endometrium, may be anechoic or hyperechoic. Most incidental, should be followed to check for propagation.
- Spontaneous abortion
- Anembryonic pregnancy, embryonic demise, blighted ovum
- Demise of a twin
- Ectopic pregnancy
- Gestational trophoblastic disease
Termination of pregnancy before 20/40 (most <12/40), spontaneous if by natural causes. Of the clinically recognised pregnancies, 25-35% undergo threatened abortion (PV bleeding, cramping with closed os) in 1st trimester, 10-15% spontaneous abortion (additional 22% in undetected pregnancies). 50% of those with abnormal 1st trimester scans abort. 70% are from fetal factors esp chromosomal abnormalities (seldom recur). 30% due to maternal factors including luteal-phase defect, physical defects (fibroids, polyps, malformations preventing implantation/support), diabetes/endocrinopathies, coagulopathies, HTN, infections (toxoplasma, Mycoplasma, Listerial, viruses; esp 2nd T), smoking, alcohol, age; may causes remain unkown. Inevitable abortion – cervical dilatation with fetal/placental tissue within os. Complete abortion – all uterine contents expelled. Incomplete abortion – RPOC in uterus. Missed abortion – foetal death, but remains in-utero. Habitual abortion – >/=3 successive spontaneous abortions.
Empty gestational sac – Embryo is not seen on US; may be very early IUP or anembryonic pregnancy/blighted ovum (embryonic death with embryo no longer visible or never developed). DDx pseudogestational sac with ectopic pregnancy. Gestational sac abnormal if any one of major criteria: absent yolk sac when MSD >/= 20, absent embryo when MSD >/= 25mm, distorted shape, growth <1mm MSD/day; or any 3 of minor criteria: irregular contour, thin decidual reaction <2mm, weak decidual echo, absent double decidual sac sign, sac low in uterus. ‘First trimester oligohydramnios’ – When MSD – CRL is <5mm, almost all spontaneously abort. Embryonic/foetal demise – absence of cardiac activity when CRL >/= 7mm on M-mode TVUS (missed abortion); if <7mm, rescan in a few days. All TA findings should be confirmed on TV.
Retained Products of Conception (RPOC)
Retained placenta/trophoblastic tissue post delivery or termination in 1%. Echogenic endometrial mass, may be vascularised, low-resistance high velocity flow (peak >20cm/s), irregular interface between endometrium and myometrium, thickened endometrium >5mm (esp after D&C, <2-5mm favours retained blood). Normal postpartum uterus commonly contains echogneic foci and fluid, gas, endometrial thickness should be <20mm. High false positive rate. If left may become infected, form synechiae or metaplasia.
Implantation outside endometrial lining of uterus, in 1:150 of all pregnancies. Typically severe abdominal pain 6/52 after LMP, correlating to rupture of the tube. High risk in Hx of PID (in 25-50% of ectopics), salpingitis, tubal surgery, endometriosis, appendicitis, ovulation induction, IVF, previous ectopics, IUCD (increased risk ~2.5x). Classicaly triad of pain, irregular menstrual bleeding, tender adnexal mass. Low, slow-rising (less than normal 2 day doubling time), or slightly reducing βhCG (rapid decrease likely miscarriage). 90% occur in fallopian tube esp isthmus/ampulla (early signs due to small diameter); interstitial/cornual ectopics develop in tube passing through uterine wall, may be large before catastrophic haemorrhage (interstitial line sign = hyperechoic endometrial line abuts mid sac rather than going around); others include ovary (?trapping of ovum within follicle), cervix (rare, below insertion of uterine aa, esp IVF), peritoneum/abdominal cavity. Heterotopic pregnancy is concomitant IUP and ectopic, esp patients taking ovulation-inducing drugs.
Confident diagnosis when see live embryo or yolk sac ectopically (in 25%), 95% probability if there is a tubal ring sign, 92% with adnexal mass, 90% with moderate/large amount of free fluid/haemoperitoneum, 43% when no IUP seen, 8% normal US, 1:30,000 when IUP confirmed in general pop (heterotopic). Tubal/adnexal ring sign (bagel/donut sign) – extrauterine gestational sac (fluid-filled) with echogenic ring (seen in 50%). Corpus luteal cyst is thin-walled cyst eccentrically from ovary, may contain clot appearing like embryo; DDx rare intra-ovarian ectopic. Haematosalpinx from ruptured ectopic – Amorphous solid/complex adnexal mass lacking embryo/sac. Decidual reaction – thickening of endometrium (but usually <8mm) by hormonal stimulation, may contain decidual cysts. Pseudogestational sac in 5-20% from fluid/blood in uterine cavity (decidual cast); elongated/beaked conforming to cavity (cf round/oval gestation sac); may show apparent double decidual sac sign, but absent/minimal peritrophoblastic flow (cf high velocity low-impedance in true sac); may contain debris mimicking an embryo. Trophoblastic tissue surrounding pregnancy (intra- or extra-uterine) is usually low-resistance (DDx malignant neovascularity, inflammation, normal progesterone phase ovaries, walls of corpus luteum).