During pregnancy, the first step is prenatal screening. These tests calculate the risk of having a child with a genetic condition and are performed as early as the 10th week of pregnancy.
Prenatal screening is offered with every pregnancy, but it is particularly important for those over age 35. Moms-to-be usually receive one of these tests:
The first trimester screening
This includes a blood test that measures two pregnancy-specific substances (known as serum analytes) in the blood; abnormal levels are linked to genetic disorders. It also includes a nuchal translucency test, which uses an ultrasound exam to measure the thickness of the clear space at the back of the unborn baby’s neck. The screening is done between 11 and 14 weeks of pregnancy.
Results are compared to those of other mothers to come up with a risk score for estimating whether a baby will be affected, says Dr. Vander Haar.
“If the risk is one in 10,000, that means the likelihood of having an abnormality is really low,” says Dr. Vander Haar. “If it’s one in 200, that’s high enough that we would recommend that you consider further testing.”
The screening is very good at assessing risk for Down syndrome and other genetic conditions, such as trisomy 13 and trisomy 18, says Dr. Vander Haar. Babies with these conditions have an extra chromosome, which results in intellectual disabilities and birth defects.
The second trimester screening
Patients who miss the window for the first trimester screening for these conditions may get a second trimester screening, a blood test also known as the quad screen because it measures four serum analytes. It’s performed between 16 and 20 weeks of pregnancy.
Some doctors perform both first and second trimester screenings and combine the results to determine an overall risk. “This combined screening can be more accurate than either screening alone, but you won’t receive results until after the second trimester test is done,” says Dr. Vander Haar.
Cell-free fetal DNA screening
Cell-free fetal DNA is a newer test that examines fetal cells found in a mother’s blood and then reports whether the pregnancy is at low or high risk for genetic conditions. This blood test is exceptionally sensitive for Down syndrome, trisomy 13, and trisomy 18.
“Overall in women over 35, it seems to have 98% to 99% accuracy,” says Dr. Vander Haar. “That means, if you have a negative result, it’s a true negative result.”
Sometimes the results of the screening aren’t positive or negative, however. In this case, “you might need a repeat blood sample, or we might move to CVS or amniocentesis,” says Dr. Vander Haar.
“If a screening test is positive,” adds Dr. Vander Haar, “there is a chance that it is a false positive, and thus we would always recommend diagnostic testing in these cases.”