For more than 30 years, Ohio State’s Maternal Fetal Medicine physicians have been performing amniocentesis. Amniocentesis is a diagnostic procedure that involves removing a small sample of amniotic fluid (approximately one to two tablespoons) from the amniotic sac. This procedure is able to diagnose greater than 99 percent of all structural and numerical chromosome changes and can detect approximately 99 percent of open neural tube defects (NTDs) by measuring levels of amniotic fluid alpha-fetoprotein (AFP). Measuring the amniotic fluid AFP values directly is more accurate than detecting the AFP levels in the mother’s blood stream (maternal serum AFP-as in quad screening). Amniocentesis can be used to test for certain genetic conditions such as Tay-Sachs disease or cystic fibrosis, depending on the patient’s situation. The test is usually performed between 16 and 20 weeks of pregnancy, though it can be performed anytime after 15 weeks.
Amniotic fluid surrounds the baby inside the mother’s uterus. It contains skin cells that the baby sheds naturally. An ultrasound is used to locate a “pocket” of amniotic fluid that is away from the baby and helps the doctor locate the safest site for inserting a needle. The needle is inserted through the mother’s abdomen and into her uterus, where one to two tablespoons of fluid will be removed. Local anesthesia is commonly used to numb the insertion site, but is not required. Most patients feel a bit of pressure or mild discomfort during the procedure, but almost all patients report that the procedure was much easier than they had imagined.
Skin cells removed from the amniotic fluid are grown so the chromosome number and structure can be examined to look for any changes. Results are typically available within 10–14 days. Since this is an invasive procedure, there is less than a half of one percent risk of complication with amniocentesis. The most serious complication is pregnancy loss, or miscarriage. This risk must be weighed against the potential benefit of the information gained from the amniocentesis. The benefit for you is something only you can decide. The risk is lowest when performed by an experienced physician.