Obstetric and Gynecologic Ultrasound
Obstetric and Gynecologic Ultrasound tests dating, viability assessment, pelvic anatomy and the safe use of Doppler. The exam rewards knowing which measurement is authoritative at each gestational age and which findings are normal variants.
Core rules
- Transvaginal gestational sac is usually visible at a serum β-hCG of about 1500–2000 mIU/mL; the discriminatory zone guides, it does not rule.
- Crown–rump length is the most accurate first-trimester dating parameter (about ±5–7 days) up to 13+6 weeks; after that use head circumference and biparietal diameter.
- Nuchal translucency is measured at 11+0 to 13+6 weeks with CRL 45–84 mm, with the fetus in a neutral position.
- Placenta previa is diagnosed in the third trimester; low-lying placenta in the second trimester often resolves by placental migration.
- Ovarian torsion can show normal arterial flow; Doppler is adjunctive, not exclusionary. Compare with the contralateral ovary and assess size, edema and peripheral follicles.
Worked example
A first-trimester scan gives CRL 35 mm. Using the rule of thumb (CRL + 42 = days), 35 + 42 = 77 days, or 11 weeks 0 days. If the last menstrual period suggests 10 weeks, the CRL wins: first-trimester biometry supersedes LMP for dating and becomes the official estimated due date. Record the measurement and the revised date so later growth assessment is against the correct baseline.
Traps that cost marks
Calling a pregnancy of unknown location before the discriminatory zone is reached; mistaking a centrally located pseudogestational sac (no double decidual sign) for a true sac; using mean sac diameter for dating when a CRL is obtainable; excluding torsion because color flow is present.