Prenatal Testing for Down Syndrome
What You Should Know About Testing for Down Syndrome
Many pregnant women want to know if they are at risk of having a child with Down syndrome and choose to undergo tests that can provide useful information. This aspect of pregnancy can be extremely stressful and is often confusing. The following information will help expectant mothers better understand Down syndrome and the tests most commonly used to evaluate a woman’s risk of having a child with this condition.
Down syndrome is a genetic disorder caused by an extra chromosome – or part of a chromosome. Children born with Down syndrome have some degree of mental retardation, which ranges from mild to moderate, depending on the child. They also have facial features that may include eyes that slant upward, small ears that fold over at the top and noses with flattened bridges, as well as other physical traits such as short stature. Heart defects, increased infections, problems with vision and hearing and other health issues are also common in children with Down syndrome. That said, children with Down syndrome usually can do most things that any young child can do, such as walking, talking and dressing themselves.
The risk of Down syndrome increases as a woman ages. A 20-year-old woman has a 1 in 1,500 chance of having a child with Down syndrome whereas a 42-year-old woman has a 1 in 55 chance. But even though older women and parents who have already had a child with Down syndrome are at greatest risk, 80 percent of babies with Down syndrome are born to mothers younger than 35. Why? Because younger women have more babies than older women.
When it comes to prenatal testing for Down syndrome, women have several options available to them, depending on where they receive their health care.
Screening tests do not provide definitive answers regarding a baby’s health. Rather, they indicate the likelihood of your child having Down syndrome. For example, a woman might be told she has a 1 in 600 chance of having a baby with Down syndrome. That means it’s unlikely that her child will have the condition, but there’s no guarantee that she’s not that one woman in 600 who is carrying a child with Down syndrome. Currently, there are two commonly used screening tests for Down syndrome: The first trimester “combined screening test” and the “quad screen,” which is given in the second trimester.
The first trimester combined screening test involves taking the pregnant woman’s blood and measuring two hormones plus using ultrasound to measure fluid behind the neck of the fetus (called “nuchal translucency”). The quad screen (typically done between 15 and 20 weeks gestation) also involves a blood test, which evaluates four components of the blood.
Sometimes screening tests indicate that a woman is at increased risk when, in fact, her baby is perfectly healthy. So if you get a troubling number, realize that your baby could be just fine. That said, many women prefer this option before or in place of diagnostic testing, such as amniocentesis, because screening is less invasive and there is no risk of harming the fetus with this approach.
Chorionic villus sampling (CVS) and amniocentesis are diagnostic tests. That means they can, with almost 100 percent certainty, indicate whether a baby has or does not have Down syndrome.
Chorionic villus sampling is performed between 11 and 13 weeks gestation. In most cases, doctors insert a thin tube through a woman’s vagina and cervix and remove a small sample of tissue, which is then sent to a lab for evaluation. Expectant mothers can feel fairly confident about the findings of this test; however, it does carry a slight risk of miscarriage. According to the Center for Disease Control and Prevention, between 1 in 200 and 1 in 100 women miscarry after CVS.
Doctors perform amniocentesis during the second trimester. Like CVS, the results are almost always correct. Using ultrasound to guide them, doctors insert a needle into the sac surrounding the baby and remove amniotic fluids for evaluation. For most women, this procedure is as uncomfortable as having blood drawn. Risks include miscarriage (1 in 300) and loss of amniotic fluid. Loss of fluid resolves itself in 80 percent of cases, but can cause complications such as infection, preterm delivery and possible pregnancy loss.
Decisions regarding genetic testing for Down syndrome are extremely personal and shouldn’t be made in haste. Rather, each expectant mother should weight the pros and cons of the options available to her and discuss them with her obstetrician.