Twin Pregnancy: Prenatal Tests in the Third Trimester

Weeks 28 through 40 mark the third trimester of pregnancy. Each week closer to full term has a great impact on the health of your babies. As the mother-to-be of twins, you may be anxious about the possibility of early labor and delivery. Set your mind at ease: Work closely with your doctor, keep all appointments, and report any symptoms of preterm labor right away.

Routine Tests

You may now be seeing your doctor every one to two weeks. This allows your doctor to keep close watch on your body as it prepares for birth. He or she can watch for any signs of preterm labor or problems with your baby or you.

Blood and urine tests. Your doctor will continue to check your urine for protein and sugar and any signs of infection, keeping a close eye for signs of preeclampsia, a complication most common in the last weeks of pregnancy. You may have blood tests again for anemia.

Other measurements. Weight, blood pressure, and fundal height measurements also continue. The babies' heartbeats are loud and clear!

Non-Stress Test

With twins, you are more likely to have non-stress tests. These tests use a fetal monitor to listen to your babies' heart rates when moving and to get some information about contractions.


Most pregnant women have just one or perhaps two ultrasounds. But you'll have this test more often, perhaps right up until birth, to check position and growth of your twins.

When needed, doctors can combine non-stress tests with ultrasound (biophysical profile). This allows your doctor to check the babies' breathing motions, body movements, and muscle tone as well as the amount of amniotic fluid.

Pelvic Exam

During the last few weeks of pregnancy, your doctor will closely monitor for changes in your cervix that might indicate labor. This process, called ripening, is when the cervix softens, thins, and opens (dilates) in preparation for labor and delivery. These changes can happen slowly or quickly during the weeks, days, or hours before birth.

Group B Strep

Doctors will routinely test for group B strep between weeks 35 to 37 of pregnancy, possibly earlier for twins. About 1 in 4 women carry group B strep bacteria in the rectum or vagina. Normally harmless in adults, these bacteria can cause your babies to become very sick if exposed during birth.

This test involves a simple swab of your vagina and rectum. The sample goes to a lab for testing. If you test positive, you'll receive antibiotics during labor so you don't pass it on to your babies.


Now What?

Be sure to get clear instructions from your doctor about what to do if you have signs of labor before your due date. Finally, along with two of everything, have extra hands at the ready for the time you are able to bring home babies.

WebMD Medical Reference Reviewed by Traci C. Johnson, MD on January 17, 2019



Lucile Packard Children's Hospital at Stanford: "Prenatal visits during the third trimester."

March of Dimes: "Routine prenatal tests" and "Fetal Fibronectin."

American College of Nurse Midwives: "Third Trimester Tests: Group B Strep Test."

The American College of Obstetricians and Gynecologists: "Group B Streptococcus and Pregnancy" and Your Pregnancy and Childbirth, Month to Month. 5th edition. 2010.

Centers for Disease Control and Prevention: "Group B Strep (GBS)."

Society of Obstetricians and Gyneacologists of Canada: "Ultrasound in Twin Pregnancies" and "Management of Twin Pregnancies."

American Association for Clinical Chemistry. Lab Tests Online. "fFn."

University of Michigan Health System: "Non-Stress Test, Amniotic Fluid Index Biophysical Profile."

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