Prenatal Tests and Procedures – Screening Tests
Screening tests measure the risk of having a baby with some genetic birth defects. Birth defects are caused by problems with a baby’s genes, inherited factors passed down from the mother and the father. Birth defects can also occur randomly in people with no family history of that disorder. Women over the age of 35 have the greatest risk of having babies with birth defects.
The benefit of screening tests is that they do not pose any risk to the fetus or mother. But screening tests cannot tell for sure if the baby has a birth defect. So, they do not give a “yes” or “no” answer. Instead, screening tests give the odds of your baby having a birth defect based on your age. Women under the age of 35 will find out if their risk is as high as that of a 35 year old woman. For women over age 35, screening tests will help them find out if their risk for their age is higher or lower than average.
Some common screening tests used during pregnancy include:
Targeted ultrasound
The best time to receive this test is between 18 and 20 weeks of pregnancy. Most major problems with the way your baby might be formed can be seen at this time. But some problems like clubbed feet and heart defects can be missed on ultrasound. Your doctor also will be able to see if the baby has any neural tube defects, such as spina bifida. This test is not the most accurate for finding out whether your baby has Down syndrome. Only 1 in 3 babies with Down syndrome have an abnormal 2nd trimester ultrasound. In most cases, your doctor can find out the sex of your baby by using ultrasound.
Maternal serum marker screening test
This blood test can be called by many different names including multiple marker screening test, triple test, quad screen, and others. This test is usually given between 15 and 20 weeks of pregnancy. It checks for birth defects such as Down syndrome, trisomy 18, or open neural tube defects. Doctors take a sample of the mother’s blood. They check the blood for 3 chemicals: alpha-fetoprotein (AFP) (made by the liver of the fetus), and two pregnancy hormones: estriol and human chorionic gonadotropin (hCG). Sometimes, doctors test for a fourth substance in the blood called inhibin-A. Testing for inhibin-A may improve the ability to detect fetuses with a high risk of Down syndrome.
Higher levels of AFP are linked with open neural tube defects. In women age 35 and over, this test finds about 80% of fetuses with Down syndrome, trisomy 18, or an open neural tube defect. In this age group, there is a false positive rate (having a positive result without actually having a fetus with one of these health problems) of 22%. In women under age 35, this test finds about 65% of fetuses with Down syndrome, and there is a false positive rate of about 5%.
Nuchal translucency screening (NTS)
This new type of screening can be done between 11 and 14 weeks of pregnancy. It uses an ultrasound and blood test to calculate the risk of some birth defects. Doctors use the ultrasound exam to check the thickness of the back of the fetus’ neck. They also test your spouse’s blood for levels of a protein called pregnancy-associated plasma protein and a hormone called human chorionic gonadotropin (hCG). Doctors use this information to tell if the fetus has a normal or greater than normal chance of having some birth defects.
In an important recent study, NTS found 87% of cases of Down syndrome when done at 11 weeks of pregnancy. When NTS was followed by another blood test done in the second trimester (maternal serum screening test), 95% of fetuses with Down syndrome were identified.
Like all screening tests, the results are sometimes misleading. In 5% of women who have NTS, results show that their babies have a high risk of having a birth defect when they are actually healthy. This is called a false positive. To find out for sure if the fetus has a birth defect, NTS must be followed by a diagnostic test like chorionic villus sampling or amniocentesis.
NTS is not yet widely used. If you and your wife are interested in NTS, talk to your doctor. If your doctor is unable to do the test, she can refer you to someone who can. Your spouse should also call her insurance company to find out if they cover the cost of this procedure. NTS allows women to find out early if there are potential health problems with the fetus. This may help them decide whether to have follow-up tests.