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Amniocentesis
If it is necessary, the amniocentesis test is usually performed for prenatal evaluation around 16 to 18 weeks of pregnancy. By this point, your uterus is large enough and there is enough fluid surrounding the baby to make the test possible. Doing the procedure at this time allows the woman enough time to make a decision about terminating the pregnancy, if that is what she desires.

With amniocentesis, ultrasound is used to locate a pocket of fluid where the fetus and placenta are not in the way. The part of the abdomen above the uterus is cleaned. Skin is numbed, and a needle is placed through the abdominal wall into the uterus. Fluid is withdrawn from the amniotic cavity (area around the baby) with a syringe. About 1 ounce (30ml) of amniotic fluid is needed to perform various tests.

Fetal cells that float in the amniotic fluid can be grown in cultures. They are the cells used to identify fetal abnormalities. We know of more than 400 abnormalities a child can be born with— amniocentesis identifies about 40 (10%) of them, including the following:
• chromosomal problems, particularly Down syndrome
• fetal sex, if sex-specific problems such as hemophilia must be identified
• skeletal diseases, such as osteogenesis imperfecta
• fetal infections, such as herpes or rubella
• central-nervous-system diseases, such as anencephaly
• hematologic (blood) diseases, such as erythroblastosis fetalis
• inborn errors of metabolism (chemical problems or deficiencies of enzymes), such as cystinuria or maple-syrup-urine disease


Risks from amniocentesis include injury to the fetus, placenta or umbilical cord; infection; miscarriage or premature labor. The use of ultrasound to guide the needle helps avoid complications but doesn't eliminate all risk. There can be bleeding from the fetus to the mother, which can be a problem because fetal and maternal blood are separate and may be different types. This is a particular risk to an Rh-negative mother carrying an Rh-positive baby. This type of bleeding can cause isoimmunization. An Rh-negative woman should receive RhoGAM at the time of amniocentesis to prevent isoimmunization.
Fetal loss from amniocentesis complications is estimated to be less than 3%. The procedure should be done only by someone who has experience doing it.