For those of you who are pregnant, sometime between 15-20 weeks of pregnancy, you will be offered maternal serum screening. What is this? And more importantly, what do the results mean? I think maternal blood screening is one of the most misunderstood tests in pregnancy. And for some women it can be a very nerve-wracking experience.
The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women regardless of age be offered a screening test for Down syndrome and certain other birth defects. This is a blood test that looks at the levels of either 3 or 4 (depending on the test) chemicals that are present in a pregnant woman’s blood. The chemicals are AFP (alpha fetoprotein), hCG (human chorionic gonadotropin—the same chemical that a home pregnancy test measures), estriol, and recently most labs have added a fourth substance, inhibin A. These chemicals are present in every pregnant woman’s blood. Some of them are produced by the fetus while others are hormones produced by the placenta.
Maternal serum tests such as the triple screen or the quad screen as they are commonly called, look at the levels of these 3 or 4 chemicals in the mother’s blood and the woman’s age, weight, race, number of fetuses (e.g. twins) and whether she has diabetes that requires insulin treatment. Then given all of those factors and the levels of the 3 or 4 chemicals the laboratory can determine the chance that a woman has a baby with Down syndrome, a neural tube defect (spina bifida), trisomy 18, or abdominal wall defects.
There are two different ways that the results may be reported. A woman may receive her test result as a ratio. For example, her baby has a 1 in 500 chance for Down syndrome. Or, in some cases, a woman’s test results are reported as normal (screen negative) or abnormal (screen positive), depending on whether her results fall below or above a cut-off point (usually about 1 in 270).
The test is NOT a diagnosis of any of these conditions though. This is where there is a lot of confusion. By definition, screening tests do not diagnose a condition. They only determine whether there is an increased risk for the condition. The good news is that of all of the women who screen positive, only a small percentage will actually have a baby with one of these disorders. Many times women have an abnormal result simply because their fetus is a few weeks older or younger than previously thought.
For women with abnormal results, the next step is usually an ultrasound. This test can check the gestational age of the fetus and show if a woman is carrying multiples. If either of these factors accounts for the abnormal test result, no further testing is needed. If ultrasound does not explain the abnormal test result, amniocentesis will be offered. An amniocentesis is an invasive test where the doctor uses a needle to remove some of the amniotic fluid around the baby. Although amnio does pose a very slight risk of miscarriage (1/4 of one percent or 1/500), it is extremely accurate and usually can give a pregnant woman a definitive answer. Should a problem exist, arrangements can be made in advance for special care of the newborn at delivery.