Prenatal diagnosis of genetic disorders associated with specific biochemical, chromosomal, or molecular characteristics can be achieved from amniotie fluid (AF) or placenta (chorionic villus: CV) samples. Chorion material is usually obtained by sampling the placenta at the implantation site, during the first trimester (i. e., 9–12 wk), using either the transcervical or transabdominal route. The first method entails access to the placenta via the cervical canal, with aspiration through a metal cannula or a flexible catheter. Alternatively, the chorionic villi may be aspirated using a needle, which is passed through the abdominal wall. In both these methods, the positioning of the aspirating device must be made with the help of ultrasound scanning. Later pregnancies can only be sampled by the transabdominal route. Amniocentesis is usually performed at 16–18 wk of gestation by the transabdominal method.
