Third Trimester Tests During Pregnancy

These tests are common in the third trimester of pregnancy:

Group B streptococcus screening: Vaginal and rectal swabs are taken at 35 to 37 weeks of pregnancy to detect group B strep bacteria. Although group B strep can be present in up to 30% of all healthy women, it's the leading cause of life-threatening infections in newborns and can also cause intellectual disability, impaired vision, and hearing loss. Women who test positive are treated with antibiotics during delivery to protect the baby from contracting the infection at birth. As an alternative, your physician or midwife may choose not to test for strep but to treat you in labor if certain risk factors develop.

Electronic fetal heart monitoring: Electronic fetal heart monitoring is done during pregnancy, labor, and delivery to monitor the heart rate of the fetus. The fetal heart rate can indicate whether the fetus is doing well or is in trouble and can be done any time after 20 weeks.

Nonstress test: Done weekly in many high-risk pregnancies, such as in cases where a women is carrying more than one fetus, or has diabetes or high blood pressure, this test involves using a fetal monitor strapped across the mother's abdomen to measure the baby's heart rate as it moves. It's also used for monitoring overdue babies.

Contraction stress test: Also done in high-risk pregnancies, a fetal monitor measures the baby's heart rate in response to contractions stimulated either by oxytocin (Pitocin) or nipple stimulation. Doctors use the measurements to predict how well the baby will cope with the stress of labor.

Biophysical profile: Can be done with just an ultrasound or with a combination of a nonstress test and an ultrasound.

WebMD Medical Reference Reviewed by Traci C. Johnson, MD on August 30, 2020


Cohen & Powderly: Infectious Diseases, 2nd edition, 2004.
WebMD Medical Reference from Healthwise: "Electronic Fetal Heart Monitoring."
WebMD Medical Reference from Healthwise: "Nonstress test."
WebMD Medical Reference from Healthwise: "Contraction Stress Test."
Gabbe: Obstetrics-Normal and Problem Pregnancies, 4th edition, 2002.

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