At the mention of the third trimester, many people tend to think of an aching back, swollen ankles, and near-constant urination. These symptoms certainly do occur in the third trimester, but there are other things to consider during this last phase of pregnancy.

pregnant woman NST - Understanding Third Trimester Screening: 5 Tests That Your Provider Might Recommend

Adobe Stock/adrian_ilie825

The third trimester is not typically known for a lot of testing, and that’s because there aren’t many routine screenings that happen during that final part of pregnancy. However, depending on your and your baby’s risk factors, your provider might suggest certain tests. Just like first and second trimester screenings, the third trimester tests could help ease fears and calm anxieties as you approach giving birth after loss.

Here are five third trimester screenings and what you need to know about them.

Group B Strep

The full name of this bacteria is group B streptococcus, but it’s usually shortened to group B strep or GBS. It’s actually common in adults and does not usually cause problems for them. In pregnant people, GBS is most often found in the vagina and rectum, meaning GBS can pass from the gestational parent to the fetus during labor and birth. It’s rare for a baby to contract GBS; only one or two babies out of 100 will. But if a baby does get GBS, it can be serious and life-threatening.

During pregnancy, your provider will recommend a GBS culture between 36 and 38 weeks. A swab is used to take a sample from the vagina and rectum, and that goes to the lab for testing. If the results show GBS is present, most birthing parents will receive IV antibiotics during labor. This helps protect the baby from contracting GBS and greatly lowers the chance of them getting sick[1]“Routine Tests During Pregnancy,” American College of Obstetricians and Gynecologists, July 2021,


Third trimester ultrasounds are usually reserved for expectant parents who go past their due date or those with high-risk pregnancies. In fact, pregnant people in the high-risk category might have multiple abdominal ultrasounds in the third trimester[2]“Prenatal Tests: Third Trimester,” Reviewed by Armando Fuentes, MD, Nemours Kids Health, August 2018,

During these scans, your prenatal care team will be checking your baby’s breathing, growth, movement, and muscle tone. They will also examine the placenta and estimate how much amniotic fluid is in the uterus.

Nonstress Test

Much like ultrasounds in the third trimester, a nonstress test (NST) is more common when there’s a need to check on the baby’s health, such as during a high-risk pregnancy or when the due date has passed[3]“Prenatal Non-stress Test,” Umana OD, Siccardi MA, StatPearls Publishing, Updated August 11, 2021, As the name suggests, a nonstress test does not put stress on the baby, but it checks to see if the fetus is responding normally to stimulation and is getting adequate oxygen[4]“Fetal Non-Stress Test (NST),” American Pregnancy Association,

During this noninvasive screening, a care team member will place a fetal monitor belt on your belly. As you recline in a chair, you’ll keep track of each movement your baby makes for 20-40 minutes. Depending on the type of fetal monitor, you might press a button every time you feel a movement. The NST results are categorized as reactive or nonreactive. Reactive suggests the baby’s heart rate and movements are in the normal range, while nonreactive means the baby didn’t meet the minimum qualifications for heart rate or movement. Nonreactive might sound scary, but it’s not a definite indicator of a problem. The next step is usually to repeat the NST for another 40 minutes, but your provider may want to do further testing.

Biophysical Profile

A biophysical profile (BPP) is an assessment of fetal well-being that often occurs when previous tests require follow-up. When you undergo a BPP, it’s usually after 32 weeks, and you’ll have an ultrasound examination and a nonstress test. Integrating the information from both of these screenings offers a more precise evaluation of your baby’s health by examining the following five markers.

  • Breathing: How the baby’s chest moves during practice breaths in the amniotic fluid
  • Heart rate: How many times the baby’s heart beats per minute
  • Movement: How often the baby moves over a certain amount of time
  • Tone: How well the baby can extend and flex limbs, like arms and legs
  • Amniotic fluid index (AFI): A measurement of how much amniotic fluid is in the uterus

Each of the five areas will receive a score of 0-2. Then, the numbers will be added together. Anything between 8-10 is considered reassuring, while a 6 could mean repeat testing or delivery if you’re full term. A score of 4 or less usually indicates further testing is necessary, or immediate or early delivery[5]“Biophysical Profile (BPP),” American Pregnancy Association,

Contraction Stress Test

Similar to the NST, a contraction stress test (CST) also measures your baby’s heart rate[6]“Prenatal Test: Contraction Stress Test,” Reviewed by Armando Fuentes, MD, Nemours Kids Health, August 2018, However, when you undergo a CST, your baby will be under slight stress. Sometimes called an oxytocin challenge test, the CST is often the next step after a concerning biophysical profile. Therefore, the CST is not as common as the NST or BPP.

During a CST, your care team will administer synthetic oxytocin, or Pitocin, or suggest nipple stimulation, both of which bring on uterine contractions. The test evaluates the baby’s heart rate as it responds to the contractions and helps your provider estimate how the baby will react to the stress of labor. During labor, blood flow to the placenta slows during contractions, but when the placenta is working properly, the baby’s heart rate remains stable. If the placenta is functioning poorly, the fetal heart rate will drop temporarily following a contraction.

Takeaways About Third Trimester Testing and Screening

As you get closer to your due date, it’s natural to have a wide range of emotions, like excitement or worry — or both. Think of third trimester tests as measures that support you having a living, healthy baby because these screenings continue to track your baby’s well-being. If you have questions or concerns, ask your prenatal care provider.

More on this topic:

Share this story!

Article Sources[+]