Obstetric and Gynecologic Ultrasound / Prenatal Diagnosis
What the exam tests
Dating, fetal anomaly screening, growth assessment, Doppler, and invasive prenatal diagnosis. The exam asks which test to use and when.
Core rules
- First-trimester scan at 11–13+6 weeks: viability, CRL dating, nuchal translucency, and early anatomy.
- Anomaly scan at 18–22 weeks: cardiac, neural axis, abdominal wall, limbs, and placental site.
- Low-lying placenta at 20 weeks is rechecked at 32–34 weeks; placenta praevia is confirmed if the os is covered.
- Amniocentesis at 15–20 weeks; CVS at 11–14 weeks. Both carry a miscarriage risk of about 0.5–1%.
- NIPT is screening; invasive testing is required for confirmation.
- Fetal growth restriction: estimated weight <10th percentile plus abnormal umbilical artery Doppler.
Worked example
A 20-week scan shows an echogenic intracardiac focus in a low-risk woman. The rest of the anatomy is normal. This is an isolated soft marker; it does not diagnose Down syndrome. Recalculate the risk; do not offer invasive testing on the marker alone. The reasoning: soft markers modify risk, but in a low-risk woman the post-test risk remains low.
Traps
Dating by last menstrual period when a scan is available; calling a low-lying placenta praevia before 32 weeks; treating NIPT as diagnostic; and ignoring Doppler in growth restriction.