Third Trimester Tests: What to Know

The third trimester runs from week 28 until birth. Your prenatal visits get more frequent, usually every two weeks and then weekly, and testing shifts too. Instead of looking for genetic conditions, most third trimester tests focus on how you and your baby are doing as birth gets closer.

Some checks are routine for everyone. Others are offered only when there's a reason to look more closely. Knowing why a test is being offered helps you understand your care and take part in decisions about it.

Which third trimester tests are routine?

• Routine for everyone: blood pressure and urine checks, blood work around 28 weeks and the Group B strep test
• Routine if you're Rh-negative: a shot of Rh immune globulin around 28 weeks
• At home: daily kick counts, starting around 28 weeks
• Only if needed: a nonstress test, biophysical profile or growth ultrasound
• Optional: cervical checks before labor

Looking for the glucose test or anatomy scan? See second trimester tests.

Routine checks and blood work

Blood pressure and urine checks

When: At every visit

Your blood pressure is checked at every prenatal visit, and your urine may be tested for protein.

What it checks for: Signs of preeclampsia, a serious blood pressure condition that most often develops in the third trimester. It can also start after birth.

When to call: Don't wait for your next appointment if you have a severe headache that won't go away, vision changes, sudden swelling in your face or hands, pain under your ribs or trouble breathing. Call your provider right away.

Third trimester blood work

When: Around 28 weeks

This blood draw is often done at the same time as your glucose screening.

What it checks for: Anemia, and for some people, repeat tests for HIV, syphilis or antibodies, depending on your health, risk and state guidelines.

Good to know: Low iron is common late in pregnancy as your blood volume increases. It can usually be treated with iron supplements.

Rh immune globulin

When: Around 28 weeks, if you're Rh-negative

If your blood type is Rh-negative, you'll usually be offered a shot of Rh immune globulin, often called RhoGAM. It keeps your body from making antibodies that could affect this pregnancy or a future one.

Good to know: You may get another dose within 72 hours after birth if your baby is Rh-positive, and after any bleeding or injury to your belly. If you're not sure whether you're Rh-negative, ask your provider.

Group B strep (GBS) test

When: 36–37 weeks

The GBS test is a quick swab of your vagina and rectum. In many practices, you can do the swab yourself.

What it checks for: Group B strep, common bacteria that live in the body and don't usually cause problems for adults. About 1 in 4 pregnant people carry it. During birth, GBS can pass to your baby and, rarely, cause a serious infection.

Good to know: If you test positive, you'll be offered IV antibiotics during labor. This greatly lowers the chance that your baby gets sick. Testing positive doesn't mean you did anything wrong, and it doesn't change your options for how you give birth.

Checking on your baby

Kick counts

When: Daily, starting around 28 weeks

Kick counts are something you do at home. Once a day, pick a time when your baby is usually active, like after a meal. Lie on your side or sit with your feet up, then count movements until you reach 10. Kicks, rolls, swishes, jabs and flutters all count. Hiccups don't.

What it checks for: Changes in your baby's usual movement pattern. Over time, you'll learn how long it usually takes your baby to reach 10.

When to call: Call your provider right away if you don't feel 10 movements in 2 hours, if your baby is moving much less than usual, or if you haven't felt your baby move all day. Babies don't slow down before birth, so don't wait until the next day.

Nonstress test (NST) — only if needed

When: Often from 32 weeks, or after your due date

During a nonstress test, two belts are placed on your belly. One tracks your baby's heart rate, and the other tracks any contractions. The test usually takes 20 to 40 minutes.

What it checks for: Whether your baby's heart rate rises when they move, which is a sign they're getting enough oxygen.

Good to know: An NST is painless and doesn't go inside your body. If your baby is sleepy, you may be asked to eat, drink or wait a little longer. It's often offered if you're past your due date, notice less movement, or have a condition like high blood pressure or diabetes.

Biophysical profile (BPP) — only if needed

When: If your provider wants a closer look at how your baby is doing

A biophysical profile combines a nonstress test with an ultrasound.

What it checks for: Five things: your baby's heart rate (from the NST), breathing movements, body movement, muscle tone and the amount of amniotic fluid. Each gets a score of 0 or 2, for a total of up to 10.

Good to know: A score of 8 to 10 is usually reassuring. A lower score may lead to more monitoring or a conversation about the timing of birth.

Growth ultrasound — only if needed

When: If there are questions about your baby's size or position

A third trimester ultrasound may be offered if there are questions about your baby's size or position, the amount of amniotic fluid, or where the placenta is.

What it checks for: Your baby's estimated size and position, amniotic fluid and the placenta's location.

Good to know: Late-pregnancy weight estimates can be off by 15% or more in either direction. If an ultrasound says your baby is measuring big or small, ask how that result would change your care and whether there's time to talk through your options.

Cervical checks — optional

When: Often offered from about 36 weeks

During a cervical check, your provider uses gloved fingers to feel your cervix.

What it checks for: How dilated (open) your cervix is, how effaced (thinned) it is, and how far your baby has moved down into your pelvis.

Good to know: Cervical checks before labor can't predict when labor will start. You could be 3 centimeters dilated for weeks, or go into labor the day after a check that showed no change. Some people find the information helpful. Others find it uncomfortable or discouraging.

A cervical check is always your choice. You can decline it, ask to wait, or ask what your provider hopes to learn. You might say:

"Can you tell me what you're hoping to learn from a cervical check, and how it would change my care?"

Questions to ask about any third trimester test

As birth gets closer, test results can start to shape decisions about induction, monitoring and the timing of birth. You deserve to understand why a test is recommended and what it means for you. You can ask:

• Why is this test being recommended for me?
• What are you looking for?
• When and how will I get my results?
• What happens if the result is abnormal?
• Would the results change my care or when I give birth?
• Is it okay to wait, or to decline?
• Is it covered by my insurance?

The BirthX takeaway

The third trimester is about keeping an eye on you and your baby as you get ready for birth. Some checks happen at every visit, and others are offered only when there's a reason to look more closely.

Knowing what each test is for helps you ask good questions, understand your results and take part in decisions about your care, right up to the day you meet your baby. Download the one-page summary to bring to your next appointment, and remember: if something feels off, you never need to wait to call.

This article is for education only and is not a substitute for medical advice. Always talk with your provider about which tests are right for you.

Sources: KidsHealth, Prenatal tests: third trimester; Children's Hospital of Philadelphia, Biophysical profile (BPP); ACOG, Prevention of Group B Streptococcal Early-Onset Disease in Newborns (Committee Opinion No. 797); UMass Memorial Health, Kick counts.