What Testing for Birth Defects Can Tell You

By Peter Van Eerden, MD Apr 03, 2017

When you are pregnant, you want to make sure everything is okay with your baby.

Major birth defects occur in 2 to 4 percent of all live-born infants. Many birth defects have an unknown cause.

Genetic conditions also contribute to a proportion of babies with birth defects. These may include structural chromosome abnormalities or a defect in one of the many genes on each of the chromosomes (known as a single gene defects).

Tests to Identify Structural Abnormalities

Ultrasounds is one of the ways the major and minor structural abnormalities can be identified in your baby. But ultrasound cannot diagnose babies with chromosome abnormalities.

Tests to Identify Genetic Conditions

Besides ultrasound, other tests are usually offered to you by your provider to screen for genetic conditions. Screening tests check to see if you are at a higher chance of having a baby with a genetic condition.

Quad Screen
Usually, in the second trimester your OB provider will offer you a screening test called the quad screen. This measures four chemicals produced by a combination of the baby and the placenta that are found in the mother’s blood.

If you are abnormally high or low, this may give you an increased risk for trisomy 21 (Down Syndrome), trisomy 18 (Edwards’ syndrome), spina bifida or problems with brain or spine formation. If that is the case, your OB provider may refer you to a maternal-fetal medicine, or high-risk pregnancy expert for further evaluation.

First Trimester Screen
Another screening test that can be done in the first trimester is called the first trimester screen. This also screens for triosmy 21 and 18. It includes an ultrasound measurement of the nuchal translucency, which is the thickness of the back of the baby’s neck, and a blood test looking at two other chemicals in the mom’s blood.

Noninvasive Prenatal Testing (NIPT)
If you are found to be a higher risk for having a genetic abnormality, you may be offered a test called noninvasive prenatal testing (NIPT). This is a newer test that we have been using for more than three years. It is a blood test done on the mom and through this we can find fetal DNA cells in maternal blood and test for abnormalities in chromosomes 21, 18, and 13 as well as the sex chromosomes.

Right now, the technology is limited to a handful of the more common genetic conditions, but in the future, we will be able to offer a blood test and look at the baby’s entire genome. NIPT has been found to be 99 percent accurate in identifying genetic conditions such as a Down syndrome with less than 0.2 percent false positive or negative rate.

Invasive Prenatal Testing Procedures
If you want more accurate information about your baby, you may choose to undergo an invasive prenatal testing procedure, which is considered to be 100 percent accurate in diagnosing trisomy 21, 18 and 13 and can tell you additional information about all of your baby’s other chromosomes.

These include chorionic villus sampling (or CVS), which is usually done in the first trimester, or amniocentesis, which is done after 15 weeks of pregnancy. These are diagnositc tests as opposed to the screening tests described above. Since these are invasive procedures

By getting as much information as you can before your baby is born, you can prepare yourself and your family for the birth of your baby. This way, there will be no surprises at the time of delivery. But this is something you must decide for yourself if the risk of the procedure outweighs the benefits.