Prenatal screening tests help identify pregnancies that are at increased risk for certain birth defects. These tests are performed in the 1rst and 2nd trimester of a pregnancy. The most common birth defects these tests are designed to identify are Down Syndrome and Trisomy 18 (both chromosomal defects) as well as open neural tube defects (also known as spina bifida, a condition that involves malformation of the spinal cord and/or brain).
It is important to remember that prenatal screening tests don’t tell you that your pregnancy is affected, rather they indicate if your risk for these birth defects is increased beyond what is expected based on your age. Whether or not you should get prenatal screening is covered in a previous post from July, 2011.
There are three prenatal screening tests. Two involve blood work alone and one involves blood work plus a special kind of ultrasound called nuchal translucency. This special first trimester ultrasound looks at the amount of fluid at the back of a baby’s neck. It involves very precise measurements and is only performed in centers with special accreditation.
The blood work involves testing the mother’s blood for proteins made by her baby. During pregnancy a small amount of the baby’s blood gets into the maternal blood stream bringing these proteins along for the ride. Higher or lower amounts of the proteins than expected (depending on which protein) is associated with certain birth defects.
There are five proteins that may be tested: human chorionic gonadotropin (hCG), pregnancy-associated plasma-protein A (PAPP-A), alphafetoprotein (AFP), inhibin-A (INH), and uncongugated estriol (µE3).
The three prenatal screening tests are as follows:
The sequential integrated screen. This test measures all of the fetal proteins found in the mother’s blood. The first blood test is performed between 10 and 13 weeks 6 days and checks the levels of hCG and PAPP-A. The second blood test is performed between 15 and 20 weeks and tests INH, AFP, µE3, as well as hCG again. A nuchal translucency ultrasound is also performed between 11 weeks 1 day and 14 weeks 2 days. This test is the most thorough and will identify approximately 90-92% of babies with Down Syndrome and probably has the best results for identifying babies with Trismony 18.
The serum integrated screen. This test involves the exact same blood work as the sequential integrated screen (first blood test between 10 weeks and 13 weeks 6 days and second blood tests between 15 and 20 weeks). The serum integrated screen identifies approximately 85-88% of babies with Down Syndrome. A serum integrated screen is typically the choice in centers where nuchal translucency ultrasound is not available.
The quad screen. This is the exact same test as the second blood test of both the sequential and serum integrated screens, meaning it is performed between 15 and 20 weeks. It measures four of the fetal proteins. A quad marker screen is typically offered when a women starts prenatal care too late to receive either the serum integrated screen or the sequential integrated screen. It has a lower detection rate than the other two tests, but still detects 77-79% of pregnancies with Down Syndrome.
All of these tests described above are superior screening tests when compared with the old triple screen and the even older alphafetoprotein testing.
Prenatal screening starts as early as 10 weeks, so that is just another reason to start prenatal care early in your pregnancy. You also want time to think about these tests and whether screening is right for you. Talk with your pregnancy provider and a genetic counsellor to get more information.
Why did I list that my references came from .gov sites? Studies show they are the most reliable from a medical standpoint and are more likely to remain current (they are maintained by medical librarians, after all). That way, if you stumble across this post in 2014 and I haven’t updated it, at least you have links that are likely to be current.