Amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus to diagnose chromosomal disorders and open neural tube defects (ONTD's) such as spinal bifid. Testing is available for other genetic defects and disorders depending on the family history and availability of laboratory testing at the time of the procedure. An amniocentesis is generally offered to women between the 15th and 20th weeks of pregnancy who are at increased risk for chromosome abnormalities, such as women who are over age 35 years of age at delivery, or those who have had an abnormal maternal serum screening test, indicating an increased risk for a chromosomal abnormality or neural tube defect.

WHEN IS IT DONE?

Amniocentesis for genetic studies is usually performed between the 15th and 17th week of gestation. Earlier procedures, called early amniocentesis, are done before 14 weeks of pregnancy. Because they are done earlier in pregnancy, results are available at an earlier gestational age but the risk of pregnancy loss is higher. Procedures performed after 20 weeks can be problematic if termination of pregnancy is planned based upon abnormal results.

Fetal lung maturity is usually assessed after 32 weeks of gestation since lung maturity is unlikely before this time.

Risks Of Amniocentesis

After an amniocentesis, women may experience cramping, bleeding, or leaking of amniotic fluid. There is also a slight risk of infection. The risk of miscarriage is about one in 200 after an amniocentesis in the second trimester of pregnancy. This is only slightly higher than the normal risk of miscarriage without an amniocentesis at this time in pregnancy.

Indirect injury to the fetus - Some studies have suggested that amniocentesis increases the risk of clubfoot, hip dislocation, and respiratory problems in the baby. These data have not been confirmed, but women should be aware of this potential risk before undergoing amniocentesis.

Infection - Studies have examined whether women who have chronic infections that could be dangerous to the fetus are more likely to transmit the infection if they have an amniocentesis procedure. Examples include HIV (the virus that causes AIDS), cytomegalovirus, hepatitis C, and toxoplasmosis. The data are not clear on whether or not there is an increased risk. Again, women should be aware that an increased risk is possible.

Benefits Of Amniocentesis

Amniocentesis helps confirm a tentative diagnosis of an abnormality found with other testing. It may also find that a fetus does not have the abnormality that was suspected. This allows couples to plan the remainder of pregnancy and to consider their options. Amniocentesis offers:

Diagnosis of a chromosomal abnormality.
Diagnosis of open neural tube defects (over 95 percent; closed or small defects may show a normal result).
Diagnosis of other defects depending on the family history and availability of testing.

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