Second-Trimester Blood Tests - dummies

By Joanne Stone, Keith Eddleman, Mary Duenwald

The following blood tests usually yield normal results, but if yours are at all unusual, you may need further testing — an ultrasound examination, perhaps. But keep in mind that further testing doesn’t necessarily mean that anything is wrong — only that your practitioner is being careful to ensure that everything is okay.

Alpha-fetoprotein screen

MSAFP stands for maternal serum alpha-fetoprotein, a protein made by the fetus that also circulates in the mother’s bloodstream. Doctors use a simple blood test to check the level of MSAFP, usually sometime between 15 and 18 weeks. The test result is affected by weight, race, and preexisting diabetes, so it has to be adjusted for those factors.

An elevated MSAFP is expressed as more than 2.0 or 2.5 multiples of the median, or MOMs, for women carrying only one baby and more than 4.0 or 4.5 MOMs for mothers of twins. (In triplets and quadruplets, the measurement hasn’t been well studied.)

Abnormal MSAFP can usually indicate whether a pregnancy is at risk for certain complications and may indicate

  • Underestimation of the fetus’s age (how far along you are)

  • The presence of twins or more

  • Bleeding that may have occurred earlier in the pregnancy

  • Neural tube defects (spina bifida, anencephaly, and others)

  • Abdominal wall defects (protrusion of the fetus’s abdominal contents through a defect in the abdominal wall)

  • Rh disease or other conditions associated with fetal edema (abnormal fluid collection in the fetus)

  • Increased risk for low birth weight, preeclampsia, or other complications

  • A rare fetal kidney condition known as congenital nephrosis

  • Fetal death

  • Other fetal abnormalities

Remember that the MSAFP test is only a screening test. Most women with an elevated MSAFP have a normal fetus and continue to have a completely normal pregnancy. Only about 5 percent of women with a positive maternal serum screen actually have a fetus with a neural tube defect. On the other hand, the test isn’t perfect, and therefore it can’t identify all abnormal fetuses.

If you have two elevated MSAFP tests, a very high single test, or a questionable ultrasound for spine and head defects, you may want to have an amniocentesis to check the level of AFP in the amniotic fluid. Your practitioner can also check the amniotic fluid for a substance called acetylcholinesterase, which is present if the fetus has an open neural tube defect. In most cases, the amniotic fluid AFP is negative and the pregnancy continues normally.

The quadruple test for Down syndrome

Another test that can be performed with the same sample of blood that’s used for the MSAFP during the second trimester is a screening test for Down syndrome — the most common chromosomal abnormality in babies.

This test can also help identify women at risk of having babies with other chromosomal abnormalities, like Trisomy 18 or Trisomy 13 (an extra copy of either the number 18 or 13 chromosome). These particular chromosomal abnormalities are associated with severe birth defects, and are often not compatible with life.

Your practitioner performs this test by measuring four substances in the blood:


  • hCG (human chorionic gonadotropin)

  • Estriol (a form of estrogen)

  • Inhibin A (a substance secreted by the placenta)

Your practitioner uses the results of these tests to calculate risk for Down syndrome. In women under the age of 35, the test detects Down syndrome in about 80 percent of the cases where it’s present. (In other words, if 100 women carrying fetuses with Down syndrome had the test, the condition would be diagnosed in about 80 of them.)

This test is only a screening, so even if the result is abnormal, the fetus is normal in the majority of cases. If your test is abnormal, your practitioner will discuss with you the possibility of having an amniocentesis to check the baby’s chromosomes.

Cell-free fetal DNA

If abnormalities are suspected on your baby’s ultrasound, your doctor may recommend sending off a sample of your blood to check the baby’s chromosomes. This test examines the cell-free fetal DNA that circulates in your blood.

Glucose screen

The glucose screen is a test to identify women who may have gestational diabetes. Your practitioner conducts the test by first having you drink a super-sweet-tasting glucose mixture (it tastes like flat soda) and then, exactly one hour later, drawing a sample of blood. He checks this sample for the level of glucose (sugar). High levels indicate that you’re at risk for gestational diabetes.

The one-hour screening test is usually performed between 24 and 28 weeks, though some doctors do it twice — once early in the pregnancy and again at 24 to 28 weeks. About 25 percent of obstetricians test only those women who are at risk for gestational diabetes.

If your initial glucose screening test is abnormal, you don’t necessarily have gestational diabetes. (Remember, it’s only a screening test.) Only about 5 percent of pregnant women actually develop gestational diabetes (although 15 percent may screen positive). Your practitioner will recommend another test that tells whether gestational diabetes is really present.

Complete blood count (CBC)

Many obstetricians check a complete blood count at the same time that they do your glucose test in order to see whether you’ve developed significant anemia (iron deficiency) or a variety of other less-common problems. Anemia is common during pregnancy, and some women need to take extra iron. A CBC also gives your physician a platelet count, which is a blood factor that helps with clotting.