Amniocentesis Risks

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Amniocentesis Risks

Question:

I am a 35-year-old female who is 19 weeks pregnant. I am concerned about whether to have an amniocentesis. My husband is adopted and knows nothing about his genetic background. My initial blood tests showed a low risk of birth defects. Do the risks of amniocentesis outweigh the benefits for someone in my situation? Are there safer tests that show chromosome defects?

Answer:

Amniocentesis is a test in which a needle is inserted into a pregnant woman's lower abdomen to remove amniotic fluid. Cells in the fluid share the same genetic make-up as the baby. These cells can show the number and structure of the baby's chromosomes and individual genes. (Chromosomes are large collections of DNA containing many thousands of genes.)

Most people have 46 chromosomes. There are some rare conditions in which people have an extra chromosome or a missing one. One example is Down syndrome. People with Down syndrome have 47 chromosomes.

The main factor affecting the risk for such conditions is the mother's age. Risk is usually not affected by the mother or father's genetic background. Being adopted does not usually affect the decision to have an amniocentesis to evaluate a baby's chromosomes.

The risk for certain conditions — such as Huntington's disease and hemophilia — are related to a variation in a single gene. This risk is affected by the parent's genetic background. That is why we say some conditions "run" in families.

Risks for these conditions may also be tied to a family's ethnic background. The gene for sickle cell anemia, for example, is more common in African Americans. And the gene for cystic fibrosis is more common in Caucasians of Northern European background.

When doctors, midwives and others collect family information during prenatal visits, they may identify genetic risks and recommend screening tests to see if the parents have the genes that can pass on diseases to their children.

Screening tests and diagnostic tests

Screening tests help judge a person's risk for a condition. Screening tests generally do not have side effects, but they also will not tell you for certain if you have a condition. Screening tests that looks for abnormal chromosomes include ultrasound measurements of a developing pregnancy and the analysis of a mother's blood for certain proteins. These tests can help judge a mother's risk for having a pregnancy affected with Down syndrome. But the screening tests can never tell for certain if a pregnancy is affected. Amniocentesis, which is a diagnostic test, can tell with near certainty if a pregnancy is affected. But an amniocentesis carries a risk of miscarriage. One out of every 400 to 600 women who have amniocentesis can miscarry.

It is estimated that our genetic make-up includes more than 30,000 individual genes. We do not have the technology to test all of these genes. Instead, individual genes are evaluated based on specific clues in the medical backgrounds of the parents. This information may be missing for people who were adopted. But this does not mean that amniocentesis is any more necessary. In fact, without knowing the family medical background of the adopted person, it is difficult to know what to test for.

It sounds as if you may have already had a screening test. And while screening tests are "safer" than amniocentesis, screening test are not as accurate as a diagnostic test such as amniocentesis.

Jeffrey Ecker, M.D. is an Associate Professor of obstetrics, gynecology and reproductive biology at Harvard Medical School and a member of the Department of Obstetrics and Gynecology at Massachusetts General Hospital, where he practices maternal-fetal medicine.

Last updated:	July 20, 2009