Medical Tests in the Second and Third Trimesters

What, When and Why

Here’s a look at some common prenatal tests and when they might be recommended during the second and third trimesters of pregnancy.

Second Trimester

Maternal blood screening (or quad screen) – Blood test to look again for potential birth defects in your baby. This test measures 4 substances in your blood. At 15–20 weeks.

Amniocentesis – A needle inserted through your abdominal wall withdraws a small sample of amniotic fluid to test for genetic conditions, such as Down syndrome and a neural tube defect (a serious condition affecting your baby’s brain or spine).

Like a CVS, amniocentesis is only recommended if you have certain risk factors, including being over age 35; having a family history of—or a previous pregnancy involving—a genetic condition; or abnormalities picked up in an ultrasound or earlier screening tests. At 15–20 weeks.

Ultrasound – Ultrasounds use sound waves to create an image of your baby on a video screen. Usually performed at least once during most pregnancies, an ultrasound can help estimate a due date, show the progress of a baby’s development or reveal if you’re having a single baby or twins. It can also usually tell you the baby’s gender.

After the sixth month, an ultrasound is used to monitor the size and position of the baby, the volume of amniotic fluid and the position of the placenta. Ultrasounds are painless, although in early pregnancy they require a full bladder to help lift the uterus into view. At 18–20 weeks.

Glucose screening – A blood test that checks for signs of gestational diabetes, a type of diabetes that some women develop during pregnancy. If you undergo this test, you will be asked to drink a concentrated sugar solution. After a short period, a blood sample will be drawn and tested to analyze how well your body reacts to the sugar.

If these test results are high, a three-hour glucose test is done to make the diagnosis. Caught early, gestational diabetes can usually be managed through diet and exercise. If you’ve had a standard gastric bypass procedure as opposed to a Lap Band your provider may want to use an alternative way to screen for diabetes. At 24–28 weeks.

Third Trimester

Antibody Screen – If an earlier blood test found that you have Rh-negative blood and your fetus has Rh-positive blood, your blood types are considered incompatible. At week 28, you may need another blood test to detect whether your body is forming antibodies against your fetus’s red blood cells. If no antibodies are found, you may receive an injection of a special blood product to help prevent them from forming. Learn more about the Rh factor in pregnancy and why it has to be managed.

Group B strep (GBS) test – Fluid extracted from your cervix is tested for signs of GBS, a type of strep that can be passed along to your baby during childbirth. At 37–38 weeks.

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