Ultrasound is used to see exactly where the fetus and placenta are located, which allows the doctor to choose the safest spot for inserting the amniocentesis needle.
The abdomen is cleansed and sometimes a local anesthetic is injected just beneath the skin. A thin needle is inserted through the abdomen and into the uterus, where one to two tablespoons of amniotic fluid are withdrawn.
After the sample is taken, an ultrasound check of the baby's heart movements is done before the woman leaves the examination room.
A few women feel cramping when the needle enters the uterus or pressure during the few minutes while the fluid is being withdrawn. Many doctors recommend that women rest for several hours after amniocentesis, avoiding physical stresses such as lifting and prolonged standing. If the cramping continues, or there is leakage or discomfort around the needle site, call the doctor.
Living cells from the woman's fluid sample are grown in the laboratory, and then tested for chromosomal abnormalities or various genetic birth defects. Results of laboratory studies of cells after amniocentesis usually take from one to three weeks and are 99 percent accurate in diagnosing and ruling out chromosomal abnormalities.
Amniocentesis is not routinely offered during pregnancy because the test itself carries a small risk of infection, miscarriage (less than 1 in every 200 women), fetal or placental injury, bleeding, premature labor and Rh sensitization.
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