First Trimester Obstetric Sonography: Normal and Abnormal
Normal milestones
Transvaginal scanning shows a gestational sac at about 4.5–5 weeks (mean sac diameter 2–3 mm), a yolk sac once the MSD reaches 8 mm, and an embryo with cardiac activity by 5.5–6 weeks. Confirm intrauterine location with the intradecidual sign and double decidual sac sign, and document adnexa bilaterally. Crown-rump length is the most accurate dating parameter between 7 and 13 weeks; GA in days is roughly CRL (mm) + 42.
Failure criteria and mimics
Pregnancy failure is diagnosed when MSD is 25 mm or greater with no embryo, or CRL is 7 mm or greater with no cardiac activity. Below those thresholds, a repeat scan in 7–10 days is required - a 5 mm embryo with no heartbeat is indeterminate, not a demise. Anembryonic pregnancy shows a sac without yolk sac or embryo, often with irregular choriodecidual reaction. A pseudogestational sac of ectopic pregnancy is central, conforms to the cavity, and lacks the double decidual sign. Exclude interstitial ectopic (myometrial mantle under 5 mm), cesarean scar implantation, and molar pregnancy (echogenic mass with cystic spaces, theca lutein cysts, markedly elevated hCG).
11–14 week assessment
Nuchal translucency is measured at CRL 45–84 mm in a true midsagittal plane, calipers on-to-on, largest measurement. Screen for acrania, holoprosencephaly, megacystis, omphalocele, and major limb defects. Physiologic midgut herniation must resolve by 12 weeks.
Rule of thumb
A yolk sac seen with a small sac and no embryo is reassuring but not diagnostic; never call a demise when MSD is under 25 mm or CRL under 7 mm. A subchorionic hematoma is described by size relative to the sac, not graded vaguely.