Ultrasound, Prenatal Diagnosis, and Genetics
Obstetric ultrasound begins with dating and viability; crown-rump length gives the most accurate dates in the first trimester. First-trimester combined screening uses nuchal translucency, free beta-hCG and PAPP-A at 11 to 13 plus 6 weeks, while cell-free DNA screening has the highest detection for trisomy 21 but remains a screening test and can be uninformative in obesity or multiple pregnancy. The anomaly scan is performed at 18-22 weeks.
Invasive testing includes chorionic villus sampling at 11-14 weeks and amniocentesis from 15 weeks, each with a procedure-related miscarriage risk of about 0.5-1%. Nuchal translucency above 3.5 mm warrants detailed anatomy assessment and fetal echocardiography regardless of aneuploidy screening results. Genetics questions test inheritance patterns - autosomal recessive 25% recurrence, autosomal dominant 50%, X-linked carrier mothers with affected sons - and the significance of consanguinity.
Traps: treating a screening test as diagnostic; quoting detection rates as if they were positive predictive values; forgetting that a normal cell-free DNA result does not exclude structural anomalies; and confusing the timing of CVS and amniocentesis.
Rule of thumb: a thickened nuchal translucency is a structural warning, not merely an aneuploidy marker.