Prenatal screening for chromosomal conditions

A 36-year-old woman has just had her first-trimester combined screen and the phone call tells her she is "high risk". She is frightened, and the registrar's first sentence matters more than any number on the page. Screening for chromosomal conditions is one of the highest-yield topics in the fellowship exam because it tests three things at once: the performance of the tests, the biology behind them, and the counselling skill that turns a risk estimate into a decision. This topic holds all three. ^[3]

Overview and definition

Prenatal screening for chromosomal conditions is the estimation, before birth, of the probability that a fetus has a chromosomal abnormality — most commonly trisomy 21 (Down syndrome), and the rarer trisomies 18 and 13. It is offered to all pregnant women in the first trimester. ^[3]

The single idea that governs everything that follows: screening estimates a risk; it does not make a diagnosis. A woman with a screen-positive result usually has an unaffected fetus. The only way to know is a diagnostic test — chorionic villus sampling (CVS) or amniocentesis — which samples fetal cells and gives a karyotype or microarray. ^[3][4]