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Prenatal Testing: Third Trimester

Medically reviewed by: Claudia Taboada, DO, FACOOG and Melanie Mitta, MD, FACOG

Overview
Prenatal Testing in the Third Trimester

Throughout your pregnancy, you'll want to know how your baby is growing. Prenatal tests in the third trimester (weeks 28–40) can offer helpful information about your health and the health of your growing baby.

Some prenatal tests during the third trimester of pregnancy are screening tests that show only the chance of a problem. Other prenatal tests are diagnostic tests that can find out if your baby has a certain problem. Sometimes you’ll have a screening test first, then a diagnostic test. Certain diagnostic tests may be done by a doctor called a maternal-fetal medicine (MFM) specialist.

Top Things to Know
  • Third trimester tests check how your pregnancy is going.
  • Your doctor will keep checking your weight, blood pressure, and urine (pee).
  • You’ll be offered a group B strep test to check for an infection that can pass to your baby at birth.
  • If you missed prenatal tests earlier in pregnancy, you might be able to have some of them now.
Do I Need to Have Prenatal Tests?

If your doctor recommends a prenatal test, ask about the risks and benefits. Most parents find that the tests put their minds at ease while helping prepare them for their baby's arrival. But it's your choice to have a test or not have it.

What Tests Can I Expect at Third Trimester Visits?

In the third trimester, you can expect to have lab tests (like a pee test) and checks of your weight and blood pressure at each visit — or almost every visit — until you give birth. The results can show if there are problems like gestational diabetes or a risk for preeclampsia, which is high blood pressure that develops in the second half of pregnancy or soon after the baby is born. You might need other tests to confirm that you have certain conditions.

Will My Doctor Offer Other Prenatal Tests?

Throughout your third trimester, you'll be offered more tests (like blood work) depending on your age, health, family medical history, and other things. The tests may include:

  • Ultrasound: This test uses sound waves to make images that show the baby's shape and position in the uterus (womb). Third-trimester ultrasounds can look at the placenta — the tissue that brings nutrients and oxygen to the developing baby. The ultrasounds are also sometimes part of a test called a biophysical profile (BPP) to see how healthy the baby is. If you have what's considered a “high-risk” pregnancy, you might have more ultrasounds in your third trimester.
  • Glucose screening:  This test checks for gestational diabetes, a short-term form of diabetes that may happen in pregnancy and can cause health problems for the baby, mainly if it’s not treated. You'll drink a sugary liquid, then have a blood test an hour later to check your glucose levels. A glucose screening may be offered in weeks 24–28 and sometimes earlier in pregnancy.
  • Group B strep test: Between your 36th and 38th weeks of pregnancy, the doctor will check you for a group B streptococcus (GBS) infection. The bacteria that cause the infection are often in the body naturally, but they can lead to serious infections in newborns. For the test, the doctor swabs the vagina and rectum (where poop is stored until it leaves the body). If you have a GBS infection, you’ll need to go the hospital as soon as labor begins so doctors can start giving you antibiotics. This medicine helps prevent your baby from getting infected.
  • Nonstress test: Doctors often do a nonstress test when they want to check on the health of your baby. This might happen if you have a high-risk pregnancy or if your due date has passed. The test sees how your baby’s heartbeat changes as your baby moves or when you have contractions (belly tightening). If your baby doesn't respond, it doesn't always mean there's a problem, but more tests might be needed, like a BPP.
  • Chorionic villus sampling (CVS): This test checks cells from the placenta (the tissue that brings nutrients and oxygen to the baby) to see if there’s a problem with chromosomes — tiny strands inside cells that carry genes. CVS tends to be done in weeks 10–13 but may be offered in the third trimester.
  • Amniocentesis: This test takes a sample of the amniotic fluid that the baby floats in to check for signs of things like chromosome problems, genetic conditions, and structural defects (problems with how the body is forming). The test is usually offered after week 15 of pregnancy.
What Else Should I Know?

Earlier in pregnancy, you may have been offered several tests in the first trimester (weeks 1–12) and second trimester. If not, you may have some of those tests in the third trimester.

If genetic tests show that your baby might have an inherited condition, it's important to talk with a genetic counselor. Your doctor can suggest one.

It's your choice if you want to have the tests or not. To decide which ones are right for you, talk with your doctor. Ask why a test is recommended, what its risks and benefits are, and what the results can — and can't — tell you.

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