The Xs and Ys of Prenatal Genetic Testing / What to know about prenatal genetic testing before making a decision

Rachel Kurian, MD
John J. Custer
February 2016 in
DallasChild, FortWorthChild
January 25, 2016
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I became a parent later in life. I didn’t marry until I was 30, after I had completed many years of school and training to become an obstetrician. So because of my profession, I was aware of risks of having a child with birth defects due to my age. As we get older, our chromosomes become “stickier,” and the risk of a piece of chromosome detaching abnormally to another, called trisomy, especially trisomy 21, 18, and 13, increases with every year. So after I suffered through two miscarriages before becoming pregnant with my first child at 33, I questioned my own reproductive success. When I finally made it through my first trimester, I had the opportunity to evaluate if my baby was healthy and normal through genetic testing. 

At the time (2007), my option was an ultrasound and blood work that gave a high detection rate for Down syndrome (up to 92 percent effective). The newest prenatal genetic screening, which became available in 2011 and is known as cell-free fetal DNA testing, that screens for Down syndrome and other genetic diseases, like extra copies of chromosomes 13 and 18 and a missing sex chromosome is even more accurate than that. A recent study published in The New England Journal of Medicine found that the new test is five and 10 times better at detecting cases of Trisomy 18 and Down syndrome, respectively, than the previous ultrasound/blood work option. And it’s still noninvasive, meaning there’s no risk to Mom or baby. That’s because the test relies on fetal fragments found in the mother’s blood. The test requires a small blood sample from Mom, typically drawn from the arm, at or around the 11th week of pregnancy, when about 10 percent of the DNA in the mother’s blood is fetal DNA from the placenta.

New genetic testing offers more accurate predictions of the risk of fetal genetic abnormalities, as well as checking Mom’s carrier status for diseases such as cystic fibrosis, spinal muscular atrophy, and Fragile X syndrome, which can cause among other things, neurodevelopmental delays, but it’s important to note that the new genetic screening is just that, a screening. It is not a diagnostic test. A positive result likely means that amniocentesis (a procedure in which a needle is inserted into the amniotic sac to get fetal cells for analysis), still the most accurate predictor of chromosomal abnormalities, or chorionic villi sampling, another invasive procedure, would be recommended.

There are other disadvantages to fetal DNA testing as well. Tests do miss some chromosomal abnormalities detected by other screening techniques, they aren’t widely covered by insurance (check with yours in advance), they aren’t regulatd by the Food and Drug Administration and for now, the American Congress of Obstetricians and Gynecologists recommends these tests for higher-risk pregnancies only — mothers 35 and older or those with a family history of genetic orders or diseases.

Suffice to say, birth defect screening and genetic testing have made amazing strides over the past 20 years. And each new year brings more technological advances on our ability to perform more precise and informative screening for all pregnancies.

So what does all of this mean for you? If you’re pregnant, share your family history and that of your partner’s with your obstetrician. Disclose all maternal medical problems, and any infectious or environmental hazards you may have been exposed to. Many birth defects are spontaneous and don’t have any mechanism for screening, as well as unforeseen events that are out of anyone’s control. I always joke with my patients that if I could wrap them all in bubble wrap and stick them in a corner for nine months, I would, but life doesn’t work like that! Above all, an open dialogue with your obstetrician is best, but only you and your partner can decide if genetic screening is the right choice for you. 


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