Amniocentesis

Why It Is Done

Amniocentesis may be done during your second trimester of pregnancy (between weeks 15 and 18) to find some birth defects when one or both parents have any of the following risk factors:

• You are older than age 35. This makes you more likely than a younger woman to have a baby with a problem such as Down syndrome.
• Either you or the father has a family history of birth defects.
• Both you and the father are known carriers of a family disease, such as Tay-Sachs disease, sickle cell anemia, thalassemia (Mediterranean anemia), or cystic fibrosis.
• You had a positive result for an integrated test. This test is a combination of several tests done in two stages at two different times during the pregnancy. A positive result may mean Down syndrome is present.
• You had an abnormal triple or quadruple screen test. The substances measured in a triple test are alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and estriol. An additional substance, the hormone inhibin A, is measured in the quadruple test.

Amniocentesis can tell the sex of your baby. This is important when you or the father may be able to pass on a disease that occurs mainly in one sex (sex-linked), such as hemophilia or Duchenne muscular dystrophy, both of which occur mainly in males.

Amniocentesis can be done during your second trimester if blood type incompatibilities are present (such as Rh sensitization). Amniocentesis will often be done at 27 weeks or before to check the severity of the sensitization by measuring the bilirubin level. The test may be repeated every 1 to 2 weeks until delivery. This will help check the health of your baby.

Amniocentesis may be done during your third trimester to:

• See if your baby's lungs are mature. This is done when your baby may need to be delivered early.
• See whether the amniotic fluid is infected (chorioamnionitis).
• Check the health of your baby if you are Rh-negative.

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Author:	Jan Nissl, RN, BS Sydney Youngerman-Cole, RN, BSN, RNC	Last Updated: October 12, 2006
Medical Review:	Renée M. Crichlow, MD - Family Medicine Siobhan M. Dolan, MD, MPH - Reproductive Genetics Liisa Honey, MD, FRCSC - Obstetrics and Gynecology
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