Amniocentesis Health Article

Definition

Amniocentesis is an optional but reliable procedure offered to pregnant women in order to obtain more information about a developing fetus in the second trimester of pregnancy. It is primarily offered to pregnant women who are at increased risk, based on their age, family history, or other factor, of having a child with a genetic condition. Amniocentesis provides accurate information about fetal chromosomes or the likelihood of certain physical abnormalities. Additional specialized studies may be performed on an as-needed basis. Women who undergo amniocentesis typically do so either to obtain reassurance about fetal well-being or, if the results are abnormal, to plan for the remainder of their prenatal care. The procedure is associated with a slightly increased chance for pregnancy loss.

Purpose

Amniocentesis has been considered a standard of obstetrical care since the 1970s. It is not, however, offered to all pregnant women. The American College of Obstetricians and Gynecologists (ACOG) recommends that amniocentesis be offered to all expectant mothers age thirty-five and older. This age cut-off has been selected because advancing maternal age is associated with an increasing risk of having a baby with a numerical chromosome abnormality. At age 35, this risk is approximately one in 200 births roughly equivalent to the risk of pregnancy loss associated with amniocentesis.

Amniocentesis is performed for the following reasons:

• Detection of structural chromosome abnormalities or an increased number of chromosomes in the fetus when the mother is in a high risk group. This includes a maternal age of 35 or more; a mother who is known to carry an abnormal chromosome such as a 21/14 translocation; family history of chromosome abnormalities; a genetic disease such as sickle cell anemia for which a DNA test is available; or abnormal triple marker screening test for Down syndrome (i.e., a low serum alpha fetoprotein or estriol or a high chronic gonadotropin level).
• Determination of the sex of the fetus when the mother is a carrier of an X-linked genetic disease such as hemophilia.
• Confirmation of an elevated maternal serum alpha fetoprotein result. Alpha fetoprotein is increased in amniotic fluid in pregnancies associated with open neural tube defects such as spina bifida and several other abnormalities that allow the fluid to leak from the fetus into the amniotic fluid.
• To evaluate the severity of hemolytic disease of the newborn (HDN). In this condition, maternal antibodies, usually anti-Rho (D), cross the placenta and facilitate destruction of Rh positive fetal red blood cells.
• Evaluation of fetal lung maturity (FLM). Respiratory distress syndrome results from collapse of the infant's lungs during expiration if the level of pulmonary surfactants is insufficient. This may occur immediately after birth especially in premature deliveries and when the mother is a diabetic. Measurement of surfactants excreted by the fetus into the amniotic fluid is used to determine the maturity of the fetal lungs.

Evaluation studies of HDN and FLM are performed in the second half of pregnancy, whereas tests for alpha fetoprotein (AFP), chromosome karyotyping, and abnormal genes are performed during the first half of pregnancy, typically between weeks 15 and 18.

Precautions

Amniocentesis is associated with a slightly increased chance of pregnancy loss (approximately 0.5%). Each woman should discuss the potential risks and benefits of amniocentesis with a doctor or genetic counselor and make her own decision about whether or not she desires this testing.

Early amniocentesis, or procedures performed before the thirteenth week of pregnancy, has been associated with an increased risk of clubfoot and of procedure-related pregnancy loss. When collecting amniotic fluid universal precautions should be followed for the prevention of transmission of bloodborne pathogens. Bloody fluid or fluid containing meconium may interfere with the tests for amniotic fluid bilirubin performed to evaluate the severity of HDN. These substances may also interfere with the interpretation of some FLM tests such as the L:S ratio (which is used to determine the level of the fetus's lung development). Some tests such as the amniotic fluid bilirubin test and alpha fetoprotein test must be interpreted with regard to gestational age.