The third trimester is less about screening for genetic or structural concerns and more about keeping close tabs on how your baby is handling these last weeks. That means checking for an infection that could pass to your baby during delivery, and watching your baby's heart rate for signs of how well they'd tolerate labor itself. Here's what these tests actually check for and why your doctor orders them.
Group B strep screening
Between 35 and 37 weeks, your doctor or midwife swabs both the vagina and rectum to check for group B strep, a common bacteria that shows up in up to about a third of healthy women without causing them any symptoms at all. The concern isn't for you, it's for your baby during delivery. GBS is a leading cause of serious newborn infections like sepsis, pneumonia, and meningitis, and in the rare case a baby develops GBS meningitis, survivors can face lasting effects like hearing loss, vision problems, or developmental delays. That sounds alarming, and it's meant to explain why the screening matters, not to worry you unnecessarily, because if you test positive, treatment is simple. You'll get antibiotics through an IV once labor starts, which works well to protect your baby. Some doctors skip the swab test entirely and instead treat based on risk factors that show up during labor, like a fever or your water breaking early, so don't be surprised if your provider's approach looks a little different.
Electronic fetal heart monitoring
This can be used any time after 20 weeks, though it comes up most during labor and delivery. A monitor tracks your baby's heart rate, which tells your doctor whether your baby is handling things well or showing signs of distress that need attention.
The nonstress test
Many high risk pregnancies, twins, diabetes, or high blood pressure among the more common reasons, get a nonstress test weekly or even twice a week in the third trimester. A belt around your belly holds a monitor that tracks your baby's heart rate as they move. It's also commonly used if you go past your due date, to keep an eye on things while everyone waits for labor to start on its own.
The contraction stress test
This one used to be a bigger part of third trimester monitoring, but it's rarely used anymore. It measures your baby's heart rate in response to contractions, either naturally occurring or brought on with a small dose of Pitocin or nipple stimulation, to see how your baby might handle the real thing during labor. The nonstress test and biophysical profile do the same basic job faster and without needing to trigger contractions, so most doctors reach for those instead now, saving the contraction stress test for situations where an earlier result was unclear.
The biophysical profile
This one pairs a nonstress test with an ultrasound, giving your doctor a fuller picture of how your baby is doing. The ultrasound checks things like your baby's movement, muscle tone, breathing motions, and the amount of amniotic fluid around them, and combined with the heart rate data, it's one of the more complete pictures your doctor can get without anything invasive.
Worth remembering. A positive group B strep result is common and very manageable, it isn't a sign anything went wrong during your pregnancy. If a nonstress test or biophysical profile comes back with results your doctor wants to watch, ask exactly what they're looking for and what the next step would be, most of the time it just means a bit more monitoring.
Getting close to your due date? Our labor checklist and guide to making a birth plan are worth going through now while you have time to think it over. If you missed it, second trimester testing covers what usually comes before all of this, and our third trimester overview has everything else changing for you and your baby right now.