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Is Prenatal Genetic Testing Right for You?

How to tell if your baby is at risk for birth defects and what tests you can take to find out for sure

by: Sydney Loney

bout two to three percent of babies have some type of major birth defect, the most common is Down syndrome. Genetic screening and diagnostic tests can help you find out if your baby is at risk. Here’s what you need to know:

Genetic screening tests vs. diagnostic tests

There are two types of prenatal testing used to detect potential fetal abnormalities: screening and diagnostic. “Screening tests do not indicate that disease is either present or absent, but tell us whether the chance of disease is higher or lower than otherwise expected for an individual patient,” says Dr. Alain Gagnon, a perinatologist at BC Women’s Hospital in Vancouver. “The purpose is to assist a pregnant woman in deciding whether or not to have more invasive, and thus risky, diagnostic testing.”

The testing you choose depends on a couple of factors, including how soon you want the results and how much risk you’re willing to take. “Tests that are done later are more precise and some women prefer the precision, but others just want the information as soon as possible,” says Dr. Gagnon. “You need to decide what kind of information you want to receive, what you’re going to do when you get it, and what risks you’re willing to take to get that information.”

The key is to talk to your doctor, ask questions and get as much information as you need. You might choose not to have any tests at all, or to have a screening test, but not a diagnostic test. Here’s some more information on the prenatal genetic tests that are available to you:

Genetic screening tests

Ultrasound

• from 11 to 14 weeks of pregnancy, a special ultrasound scan called a nuchal translucency test (NT) can determine your risk of having a baby with Down syndrome
• the ultrasound measures the fluid under the skin at the back of the baby’s neck (all babies have some fluid, but in many babies with Down syndrome, the nuchal translucency is increased)
• done alone, the test picks up about 75 percent of babies with Down syndrome, but when combined with a blood test, the detection rate improves to about 90 percent
• results can take up to a week
• there is no risk to the baby

Maternal Serum Screen (double screen, triple screen or quadruple screen)

• usually done at 15 to 16 weeks
• these blood tests look for certain “markers” for Down syndrome in the mother’s blood (a double screen looks for two markers, a triple screen looks for three, etc.)
• results are available in one to two weeks
• when it comes to accuracy, the double test detects about 59 percent of babies with Down’s, and the quadruple detects about 75 percent
• MSS testing doesn’t actually show if a baby has a chromosomal problem, but instead gives an estimate of risk
• only two or three out of every 100 women who have an abnormal MSS will have a child with birth defects (an incorrectly estimated due date makes your hormone levels appear abnormal, which is a common cause of false results)

Integrated Test

• an alternative to MSS, Integrated Screening combines nuchal translucency scanning and a blood test in the first trimester with a further blood test for additional markers in the second trimester
• detection rate is about 94 percent for Down syndrome, although it means waiting until your second trimester to get the results

Diagnostic tests

If screening tests show your baby could be at risk for a birth defect, you may decide to undergo a more invasive diagnostic test. You may also decide to have a diagnostic test if there’s a history of birth defects in your family, or if you’re over the age of 35–the risk of Down syndrome and other chromosomal problems increases with the mother’s age. (At 35 years, the risk of having a baby with Down’s is about one in 200.)

Chorionic Villus Sampling (CVS)

• this diagnostic test is offered around 10 weeks of pregnancy to identify genetic disorders
• a needle is inserted into the womb through the abdomen or vagina and a sample of tissue from the placenta is removed
• results take one to two weeks
• unlike amniocentesis, CVS doesn’t detect neural tube defects, such as spina bifida
• the risk of miscarriage from CVS is about one in 100

Amniocentesis

• a diagnostic test that is usually offered at about 15 weeks gestation, when there is enough amniotic fluid to protect the fetus and reduce the risk of miscarriage
• a needle is inserted into the mother’s abdomen and a sample of amniotic fluid is removed and tested for chromosomal abnormalities
• amniocentesis can detect both genetic disorders and neural tube defects
• results are available in about two to three weeks
• the risk of losing the pregnancy due to an amniocentesis is about one in 200

What to do if the tests are positive

“The biggest misconception is that having a screening or diagnostic test means that you want to have a termination,” says Dr. Gagnon. “For many women, it’s about preparation and optimizing the place where the baby will be born and the care at time of delivery. Often an advantage to knowing in advance is you have time to plan and get prepared.” Once you know your risks, it might help to speak to a genetic counsellor to review your options and to help you decide on the best course of action for you and your family.

Meet our expert:

Dr. Alain Gagnon is a perinatologist at BC Women’s Hospital and Health Centre in Vancouver.