We have long known that prenatal care, the global care of a pregnant woman and her developing baby, leads to safer deliveries and healthier babies. As you visit your obstetrician monthly, then more often over the course of your pregnancy, we look for potential problems that may threaten your pregnancy outcome. Screening tests to identify certain birth defects have become a routine part of prenatal care.
What is Prenatal Screening
Prenatal screening refers to our efforts to determine the risk that the developing fetus would be affected by certain disorders that could dramatically affect its health or survival. We feel that this important information should be offered to all expectant parents. We understand that individual parents have differing feelings about whether they would want to have such a risk analysis. Some parents want what information is available, which for the majority provides reassurance. But others do not feel that they would benefit from having this information during pregnancy, and prefer to deal with whatever challenges they may face after their baby’s birth and beyond.
Our discussion here will focus on the prenatal screening available to detect certain chromosome defects. Each organ or human body part is made up of cells. Every cell contains genes that direct how the cell functions. Our genes control the work of each body part by controlling its individual cells. Defective genes may direct cells to function abnormally or not at all. This results in a genetic disease.
What are chromosome abnormalities?
Our genes are grouped inside the cell on structures called chromosomes. Each cell contains 46 chromosomes arranged in 23 pairs. If a chromosome is missing, broken, or if there is an extra chromosome (all types of chromosome abnormalities), the genes on it may malfunction.
Rarely, one of the chromosome bundles is actually a group of three instead of a pair—thus the word “trisomy.” The mistake has occurred during either sperm or egg production. The defective sperm or egg may be the one to randomly participate in the conception of the new pregnancy. Most often the defect occurs within the 21st chromosome group. The resulting condition is Trisomy 21, also known as Down Syndrome. Babies with Down Syndrome have varying degrees of intellectual handicap and medical problems. Trisomies 18 and 13 occur much less frequently and are not compatible with life after birth.
What tests are available?
I would like my patients to have a general understanding of the tests we have available to screen for certain specific chromosome abnormalities during pregnancy. We do not have the ability to screen for all genetic disorders, or even for all chromosome defects.
Screening tests are available to determine the risk that an individual fetus has Trisomy 21. (Tests may also give risk assessment for Trisomies 18 and 13.) A woman’s risk of having an infant with Down Syndrome increases significantly after age 35. Nonetheless, most infants with Down Syndrome are born to mothers younger than 35. This is because most babies are born to women under 35.
Screening during the first or second trimester allows us to use a more sophisticated analysis than the mother’s age alone to determine which pregnancies may have increased risk for Down Syndrome. We offer this risk analysis to all pregnant patients. It consists of a combination of blood tests and ultrasound examination. The test identifies those mothers who are at high risk to have a child with Down Syndrome regardless of their age (defined as risk equal to or greater than that of the average 35 year old). With the screening test, a 20 year old mother may find out that her risk is similar to that of the average 40 year old.
In the past, when the screening test detected high risk, we would offer that mother an invasive procedure to determine with a very high degree of accuracy whether her developing fetus had Down Syndrome. These procedures, (amniocentesis or chorionic villi sampling) involve inserting a needle into the uterus to obtain actual fetal cells from the amniotic fluid or placenta) and carry a small risk of harming the pregnancy. Recently, newer tests have become available that are very accurate in detecting chromosome abnormalities early in pregnancy and require only a blood sample from the mother. The new “cell-free” fetal DNA tests are only recommended for women who are high-risk either because of their age or based on the results of their first or second trimester screening tests. They are not recommended for women of average risk or women with multiples because they have not been sufficiently evaluated in those groups.
While the new test allows us to give families accurate information without risk to the pregnancy, it still has limitations. The test will not identify 100% of chromosome abnormalities. It also carries a very small risk of falsely identifying a chromosome abnormality. For these reasons, we refer our high-risk patients who have chosen to be tested to a specialist in maternal-fetal medicine. We are fortunate here at Brookwood to have on staff Dr. Tony Gonzalez. Dr. Gonzalez, an experienced, board-certified maternal fetal medicine specialist, is able to provide this testing, as well as specialized ultrasound or more invasive testing if necessary, in a private-practice setting. Most important, he is able to support patients with appropriate counseling about the meaning of the test results for their pregnancy outcome.
We encourage you to consider your risk factors and personal feelings about genetic testing early in your pregnancy. We are available to answer any questions you may have.
Read more general information in the American Congress of Obstetricians and Gynecologists’ patient brochure “Screening for Birth Defects.”