Amniocentesis is a test to detect abnormalities in the baby. It may also be done to assess fetal lung maturity if induction of labor or cesarean delivery is indicated. The test is performed by administering a local anesthetic, then drawing a sample of amniotic fluid using a long needle inserted through the abdominal and uterine walls. It should be done in conjunction with ultrasound, which will show the location of the fetus, the placenta, and the umbilical cord.

The amniotic fluid is sent to a lab for culturing and testing. Certain tests such as those for Tay-Sachs disease, Hunter’s syndrome, neural tube defects, and fetal lung maturity can be performed immediately. These tests measure chemicals produced by the baby that are present in the amniotic fluid. To determine fetal lung maturity, the amniotic fluid is examined for the specific phospholipids that are present when a baby’s lungs are mature.

Most tests require that the fetal cells be isolated and given sufficient time to multiply, usually 2 to 4 weeks. Then a complete chromosomal analysis is done to detect the presence of Down syndrome or other genetic abnormalities. The sex of the baby is also established. When amniocentesis is used for detecting abnormalities, it is generally performed between 16 and 18 weeks gestation. Before 14 weeks, there usually is insufficient amniotic fluid. Some caregivers are performing amniocentesis between 11 and 13 weeks. But one study found that when amniocentesis is performed before 86 days, an increase in foot deformities (turned-in and clubfeet) was observed. When testing for gestational maturity, the procedure is done in the last trimester.

Amniocentesis is a very valuable procedure, but according to many experts, it can be misused. The risks include some chance of blood exchange between the woman and baby, creating a greater Rh incompatibility when testing for the condition; infection of the amniotic fluid; peritonitis; blood clots; placental hemorrhage; injury to the baby; and even premature labor Amniocentesis should definitely not be done just to satisfy curiosity about the baby’s sex. And, unless you plan to terminate your pregnancy if an abnormality is found, or unless your doctor plans to alter your care, it is questionably whether you should have this test done just for the genetic information.