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| |    Amniocentesis for Second Trimester Diagnosis by Marjorie Greenfield, M.D. reviewed by Laura Jana, M.D., F.A.A.P. The procedure Amniocentesis is a procedure to remove some amniotic fluid from around the baby. Amniocentesis is done with ultrasound guidance, using a long needle to reach through the mother's abdomen into the amniotic sac. Although this procedure sounds like it would be painful, it hurts only a bit more than getting blood drawn, and less than getting a shot, or having an IV started. Amniocentesis is usually done around 16 weeks gestation, although sometimes can be done earlier in pregnancy. Amniocentesis offers a look at fetal chromosomes The fluid that is obtained at amniocentesis contains many fetal cells. The chromosome content of these cells can be analyzed in the laboratory. In Down Syndrome, and in some other conditions, there is an abnormal number of chromosomes in each cell. Amniocentesis is very accurate in identifying these sorts of conditions. There are now also many tests for more subtle genetic conditions. Genetic counseling before an amniocentesis can determine which tests should be done on a case-by-case basis. Alpha feto-protein measurement Amniocentesis fluid can also be analyzed for alpha feto-protein (AFP). High levels of AFP can be a sign of physical malformations of the fetus, in which there is an abnormal opening to a body cavity, such as the spinal canal. There is a blood test to measure AFP (see Triple Check), but amniocentesis is more accurate. Women who are going to have amniocentesis do not need the triple check test: the amniotic fluid gives even better information. Risks of amniocentesis When done in the second trimester, amniocentesis has a small risk of causing a miscarriage. Ask your practitioner what the risk is in your institution. Usually that risk is about 1 in 200, meaning that 99.5% of amniocentesis tests do not cause fetal loss. Who should have amniocentesis? The risk of Down Syndrome slowly increases with the mother's age. At 35 years old, women have a risk of about 1 in 200 of giving birth to a baby with Down Syndrome. In the United States, the standard is to offer amniocentesis to all women who have more than a 1 in 200 chance of identifying a genetic problem in the fetus, that is, when the risk of Down Syndrome equals the risk of causing a miscarriage with the amniocentesis. Of course, this represents a value judgement that does not hold for all families. For some, any risk of Down Syndrome feels unacceptable, and they choose to have amniocentesis even though the risk of the test causing a miscarriage is higher than the risk of Down Syndrome that the test might detect. For others, the risk of causing the loss of an otherwise healthy pregnancy feels unacceptable, and they choose to decline amniocentesis, even if they are at high risk of finding a problem. This is often the case with older moms who have gone through infertility treatment, who feel they cannot risk losing the pregnancy under any circumstances. Alternatives to amniocentesis There are other tests that can assess the fetal chromosomes. Chorionic villi sampling (CVS) can be done earlier than amniocentesis (around 10-11 weeks of gestation), but may carry a slightly higher risk of causing a miscarriage. It is particularly useful in cases where there is a high likelihood of finding a genetic problem, such as in couples who both carry a serious recessive genetic trait, giving their baby a 25% chance of being affected. If you think that CVS may be for you, discuss this alternative to amniocentesis with your practitioner. Research is currently underway to find a method to extract fetal cells from the mother's blood. If this becomes possible, under many circumstances a simple no-risk blood test could be substituted for amniocentesis.
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