This measurement is used as a screening test to assess the risk for certain chromosome abnormalities, such as Down Syndrome. Because it is only a screen, an abnormal result simply means there is a higher than normal chance for chromosome problem. Further testing is required to make the specific diagnosis. Ultrasound is used with the procedures of the amniocentesis and chorionic villus sampling to find the best location for sampling and for guidance of the instruments needed for collecting the cells. During amniocentesis, the fetus and the amniotic fluid are visualized by ultrasonography to guide the needle placement.
Amniotic fluid contains loose cells shed by the skin of the fetus.
The fluid sample is delivered immediately to the laboratory where the cells are grown into a culture sufficiently large for diagnostic tests. Chromosomes are analyzed and chemical assays and DNA studies are performed as indicated. Cells are withdrawn from the placenta. Ultrasonography is used to guide placement of the catheter or needle. Early amniocentesis may also be possible between 11 and 14 weeks of pregnancy.