Prenatal Tests: Third Trimester
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Prenatal Tests: Third Trimester

Reviewed by: Armando Fuentes, MD

Throughout your pregnancy, you'll want to know how your baby is growing. Prenatal tests can offer valuable information about your health and the health of your growing child.

If your doctor recommends a screening or test, be sure to learn about the risks and benefits. Most parents find that prenatal tests offer them peace of mind while helping to prepare them for their baby's arrival. But it's your choice to accept or decline a test.

Routine Screenings & Other Tests

Your first visit to the obstetrician should have included a full physical, urine (pee) test, and blood test to check for things like:

  • your blood type and Rh factor. If your blood is Rh negative and your partner's is Rh positive, you may develop antibodies that prove dangerous to your fetus. This can be prevented through an injection given around the 28th week of pregnancy.
  • anemia, a low red blood cell count
  • hepatitis B, syphilis, and HIV
  • immunity to German measles (rubella) and chickenpox (varicella)
  • cystic fibrosis and spinal muscular atrophy. Health care providers now routinely offer to screen for these disorders even when there's no family history.

You can expect to get your urine tested and your weight and blood pressure checked at every (or almost every) visit until you deliver. These tests can identify conditions such as gestational diabetes and preeclampsia (dangerously high blood pressure).

Throughout your third trimester, you'll be offered more tests depending on your age, health, family medical history, and other things. These can include:

  • Ultrasound: An ultrasound is a safe and painless test that uses sound waves to make images that show the baby's shape and position in the uterus. Third-trimester ultrasounds can examine the placenta, and sometimes are part of a test called a biophysical profile (BPP) to see whether the baby is getting enough oxygen. Women with high-risk pregnancies may have multiple ultrasounds in their third trimester.
  • Glucose screening: This test checks for gestational diabetes, a short-term form of diabetes that develops in some women during pregnancy and can cause health problems for the baby, especially if it is not diagnosed or treated. You'll drink a sugary liquid, then have a blood test an hour later to check glucose levels.
  • Group B strep test: Between your 35th and 37th weeks of pregnancy, the doctor will check you for group B streptococcus (GBS) infection. GBS bacteria are found naturally in the vaginas of many women but can cause serious infections in newborns. This test involves swabbing the vagina and rectum. A woman whose test comes back positive must go to the hospital as soon as labor begins so that intravenous (IV) antibiotics can be started to help protect the baby from becoming infected.
  • Nonstress test: A nonstress test (NST) is usually done when a health care provider wants to check on the health of the fetus, such as in a high-risk pregnancy or when the due date has passed. The test checks to see if the baby responds normally to stimulation and is getting enough oxygen. A baby that doesn't respond isn't necessarily in danger, but more testing might be needed.
  • Contraction stress test: This test stimulates the uterus with pitocin, a synthetic form of oxytocin (a hormone secreted during childbirth), to determine the effect of contractions on fetal heart rate. It may be recommended when an earlier test indicated a problem and can see whether the baby's heart rate is stable during contractions.

Remember that tests are offered to you — it's your choice whether to have them.

To decide which tests are right for you, talk with your health care provider about why a test is recommended, its risks and benefits, and what the results can — and can't — tell you.

Reviewed by: Armando Fuentes, MD
Date reviewed: August 2018