Second, some problems do not yet have a detection test. It is usually done at 15-18 weeks of pregnancy. It is performed in a hospital or in the health care provider's office. You do not need to stay overnight. The Procedure A sample of amniotic fluid is withdrawn through a needle from the amniotic sac that surrounds the fetus .
Amniotic fluid contains cells from the fetus that can be tested.
These cells have the same genetic makeup as the fetus. For the procedure, the patient lies down and her abdomen is uncovered. First, ultrasound is used to show the health care provider where to insert the needle to avoid touching the fetus. The needle is then carefully guided through the abdomen and the uterus into the amniotic sac. If you are carrying twins, the health care provider will need to take a sample from each sac.