chorionic villus sampling

Chorionic villus sampling tests a pregnancy by examining a tiny piece of the placenta. Because the placenta grows from the same fertilized egg as the fetus, its cells usually share the baby’s genetic makeup, making them a useful stand-in for testing.

Guided by ultrasound, a clinician takes a small sample of the chorionic villi — fingerlike projections of placental tissue — either through the cervix or the abdomen. Its main advantage over amniocentesis is timing: it can be done earlier in pregnancy, in the first trimester, giving families answers sooner.

Like amniocentesis, it is invasive and carries a small risk to the pregnancy. One technical caveat is that the placenta can occasionally carry a chromosomal change confined to it and not present in the fetus, so an ambiguous result may need confirmation by amniocentesis later.