Major birth defects occur in 2% to 4% 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 a baby. But ultrasound cannot diagnose babies with chromosome abnormalities.
Tests to identify genetic conditions
Besides ultrasound, other tests are usually offered to screen for genetic conditions. Screening tests check to see if parents are at a higher chance of having a baby with a genetic condition.
Usually, in the second trimester OB providers will offer 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 the results are abnormally high or low, this may increase the risk for trisomy 21 (Down Syndrome), trisomy 18 (Edwards’ syndrome), spina bifida or problems with brain or spine formation. If that is the case, the OB provider may refer the mother 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 trisomy 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 mothers are found to be a higher risk for having a genetic abnormality, they may be offered a test called noninvasive prenatal testing (NIPT). This is a newer test that has been used for more than three years. It is a blood test done on the mom which 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, will be able to offer a blood test and look at the baby’s entire genome. NIPT has been found to be 99% accurate in identifying genetic conditions such as a Down syndrome with less than 0.2% false positive or negative rate.
Invasive prenatal testing procedures
For more accurate information about their baby, parents may choose to undergo an invasive prenatal testing procedure, which is considered to be 100% accurate in diagnosing trisomy 21, 18 and 13 and can tell additional information about all of the 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 diagnostic tests as opposed to the screening tests described above. Since these are invasive procedures
By getting this information before the baby is born, mothers can prepare themselves and their family for the birth of their baby. This way, there will be no surprises at the time of delivery. However, this is something mothers must decide for themselves if the risk of the procedure outweighs the benefits.
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