Although researchers have pinpointed how Down syndrome occurs, they still don't know very much about why it happens. This can make it difficult to understand whether you are at risk of having a baby with Down syndrome. "There have been theories about whether it's due to how well the mother metabolizes folate, but there are just as many studies saying no as studies saying yes," explains Kenneth Rosenbaum, M.D., founder of the division of genetics and metabolism and co-director of the Down Syndrome Clinic at Children's National Medical Center in Washington, D.C. "For the most part, we simply don't understand this as well as we would like," he says.
Down syndrome is a condition that happens when a baby is conceived with a full or partial extra copy of chromosome 21. In about 3 percent to 4 percent of children with Down syndrome, there are two full 21st chromosomes and a partial extra chromosome 21 stuck to a different chromosome altogether, the result of a process called translocation. Down syndrome occurs all around the world, in people of every race and economic background. And although it is a genetic condition, you most likely will not have any family history. "In most cases, the extra chromosome seems to happen by chance," says Emily Jean Davidson, M.D., clinical director of the Down Syndrome Program at Boston Children's Hospital. Down syndrome can occur at any maternal age, but the possibility increases as a woman gets older. That said, if you have one child with Down syndrome, your chance of having a second child with the condition is about 1 percent for a mother over 40. If your baby has a translocation, physicians will suggest checking both parents' chromosomes to see if the translocation arose in the baby (most commonly the case) or if either parent is a carrier. If so, genetic testing can pinpoint the cause and determine your individual chance of recurrence.
But there is one key risk factor for Down syndrome: maternal age. A 25-year-old woman has a 1 in 1,200 chance of having a baby with Down syndrome; by 35, the risk has increased to 1 in 350; by age 40, to 1 in 100; and by 49, it's 1 in 10, according to the National Down Syndrome Society. If you are over 35 and thinking about getting pregnant, you and your partner may want to undergo genetic counseling to more precisely pinpoint your risk.
Regardless of your age, the American College of Obstetrics and Gynecology recommends that all pregnant women should be offered prenatal genetic screening for Down syndrome and other genetic conditions. During weeks 11 to 14 of your pregnancy, a blood test combined with an ultrasound (which checks the thickness at the back of the fetus's neck, known as its nuchal translucency) can detect Down syndrome in 82 to 87 percent of cases, with minimal risk to you or your baby. If you are over 35 or have other known risk factors, your doctor may also suggest a fetal DNA test (sold commercially as MaterniT21 or Harmony) during the first trimester; it has a 99 percent accuracy rate because it works by sequencing the small pieces of your baby's DNA that circulates in your bloodstream during pregnancy. In your second trimester, another blood test, known as the multiple marker screening, or quad screening (because it checks the level of four different substances in your blood) can detect Down syndrome with 80 percent accuracy. If any of these screening tests raise a red flag, you can have a diagnostic test such as amniocentesis or chorionic villus sampling (CVS), though these procedures do carry a small risk of miscarriage. Deciding whether to be screened, and which tests to have, is a personal choice: "Some women feel that the more information they have, the better they can prepare themselves, or they may make some tough decisions," says Dr. Davidson, noting that diagnosing Down syndrome in utero does help doctors know to keep close tabs on whether your baby has any potentially life-threatening related issues, such as congenital heart disease. "But some families decline being screened because they feel the information wouldn't change the course of their pregnancy."
No matter what your risk, remember that Down syndrome occurs before conception: "Nothing you do during your pregnancy will increase your risk or reverse what has already happened," Dr. Rosenbaum says. "All you can do while pregnant is take the best possible care of yourself and your baby by eating well, taking prenatal vitamins, and following other common sense advice." If you do receive a prenatal diagnosis of Down syndrome, the National Down Syndrome Society (ndss.org) has good advice for expectant parents; lettercase.org and downsyndromepregnancy.org are other useful resources.
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