Chorionic Villus Sampling

When e.g. a high combined risk for Down syndrome is found at the first trimester screening, a chorionic villus sample is offered. Chorionic villi are part of the placenta and serve as the border between maternal and fetal blood during pregnancy. The cells of the chorionic villi resemble the cells of the fetus because a proportion of them have been derived from the fertilized egg.

A chorionic villus sample is obtained with a fine needle through the abdominal wall, under guidance of ultrasound. The procedure has an associated risk of miscarriage of approximately 1%.