Prenatal Tests in the 3rd Trimester

Weeks 28 through 40 mark the third trimester of pregnancy. The big goal now is to carry baby to term. You can expect more frequent prenatal visits, but usually less in the way of lab testing.

Routine Tests

As the weeks tick by, you'll begin to visit your doctor more often -- every two to three weeks starting around week 28, and then moving to weekly at week 36. This allows your doctor to keep close watch on your body as it prepares to go into labor. He or she can note 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 that is 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. Baby's heartbeats are loud and clear!

Pelvic Exams

In the last few weeks of pregnancy, your doctor will start doing pelvic exams again. This is to see if the cervix has started the ripening process for birth. Ripening is the softening, thinning, and opening (dilating) of the cervix.

These changes can happen slowly or quickly during the weeks, days, or hours before birth. So, it's not uncommon to dilate a few centimeters a few weeks before your due date and then to stop dilating. This process is somewhat unpredictable. 

Group B Strep

Doctors routinely test for group B strep between weeks 35 to 37 of pregnancy. That's because about 1 in 4 women carry group B strep bacteria in the rectum or vagina. Although normally harmless in adults, these bacteria can cause your baby to become very sick if exposed during birth.

The 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 baby.

Now What?

During your last trimester, you might consider taking childbirth education classes at your local hospital. Classes can help you physically and mentally prepare for what is to come during labor and delivery. Also be sure to get clear instructions from your doctor about what to do if you have signs of labor before your due date.

WebMD Medical Reference Reviewed by Trina Pagano, MD on January 16, 2019



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

March of Dimes: "Routine prenatal tests."

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.

CDC: "Group B Strep (GBS)." 

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