Tests Performed Only During Pregnancy
First Trimester Screening
This test is done during pregnancy between 11 and 14 weeks of gestation. It combines a blood test for hCG (the pregnancy hormone) and a specific protein (pregnancy associated plasma protein A or PAPP-A) with an ultrasound measurement of the back of the baby’s neck (nuchal translucency or NT). While there is a range of values for these in normal pregnancies, women with higher than average NT measurements, or higher or lower than average hCG or PAPP-A values may be at risk for having a baby with Down syndrome or another rare chromosome problem called Trisomy 18. The first trimester screen cannot tell for certain whether the fetus does or does not have these problems, but it can identify women who have a greater than average risk. About 4 or 5 of every 100 women who have first trimester screening will be offered further testing by amniocentesis or CVS, but most of the time those women will deliver a normal, healthy baby.
Quad Screening (2nd Trimester)
The Quad Screen is done during pregnancy usually between 15 to 18 weeks of gestation. It measures levels of four substances (alpha-fetoprotein (AFP), hCG, unconjugated estriol and dimeric inhibin) in a pregnant woman’s blood. These substances come from the fetus and placenta and are found in the blood of all pregnant women. It is normal for the levels of these substances to vary among different women, but some women with particularly high or low values of the various substances may be at increased risk for having a baby with Down syndrome, Trisomy 18 or an opening in the babies body caused by a neural tube defect (NTD) or an abdominal wall defect. The Quad Screen cannot tell for certain whether the fetus does or does not have a birth defect, but it can identify women who have a greater than average risk. About 7 or 8 of every 100 women who have AFP+ testing will be offered further testing, but most of the time those women will deliver a normal, healthy baby.
Ultrasound uses sound waves to create pictures of the baby before it is born. Most of the organs and bones of the baby can be seen with ultrasound. This technology is used early in the pregnancy to establish how far along a pregnancy is, or to make sure an early pregnancy is viable. Nuchal translucency measurements for the 1st Trimester Screen (above) are done at 11-14 weeks of pregnancy, and detailed evaluation of the baby’s anatomy is usually done between 18 and 20 weeks. Also at 18 to 20 weeks, most ultrasound exams include a “screen” for subtle indications (“soft signs”) that may indicate an increased risk for Down syndrome. Some of these include skin swelling at the back of the neck, slightly shorter than average upper arm or upper leg bones, or a small bright spot (echogenic intracardiac focus, EIF) seen on the ultrasound of the baby’s heart. There are no known risks from ultrasound.