Pregnancy: 7 Common Third Trimester Tests

During the last trimester of pregnancy your doctor may recommend that you have certain diagnostic tests. These tests are all safe and have been developed to ensure the optimum well-being of both mother and baby.

Listed are some of the most common tests women who are pregnant will given.

Group B streptococcus (GBS) is a type of bacteria that can exist in the female reproductive tract without causing symptoms. Up to 30% of healthy women can carry group B strep, and it usually does not cause problems. However, sometimes it can lead to serious infection of the bloodstream, infection of the placenta, or urinary tract infection. Group B strep can also have serious consequences for the baby, causing potentially life-threatening infections in the newborn, including meningitis, pneumonia, and sepsis. Testing is simple and involves taking a sample from the vagina and from the rectum with a cotton swab. The swab is cultured in the laboratory to determine if group B strep is present. If the test is positive, you will be given antibiotics during labor to reduce the chances of infection in the baby. The test is usually done between the 35th and 37th weeks of pregnancy and should be done in subsequent pregnancies even if you test negative in your first pregnancy.

Electronic fetal heart rate monitoring is often done to confirm that the baby is healthy. This type of monitoring is also done during labor and delivery. It can be done any time after the 20th week of gestation during prenatal checkups.

The nonstress test (NST) involves a fetal monitor strapped to the mother's abdomen to measure the baby's heart rate as it moves. It is called "nonstress" because no stress is placed on the fetus for the test. This test is sometimes performed on a weekly basis in high-risk pregnancies. It is done after the 28th week of pregnancy, though most often after 32 weeks. Measurements are typically taken for 20 to 30 minutes. A NST may be ordered if you feel the baby is not moving normally, if you are past your due date, or if your doctor wants to ensure that the placenta is healthy and functioning well. There are no known risks of the NST for the mother or the baby.

A biophysical profile combines the information from a NST with an ultrasound examination of the baby for a more precise evaluation.

A contraction stress test (CST), like the NST, measures fetal heart rate. However in this test, the baby's heart rate is measured in response to uterine contractions that are elicited by administering oxytocin (Pitocin) or by stimulation of the nipples. The test is sometimes referred to as an oxytocin challenge test. Normally, the flow of blood to the placenta slows during contractions, but if the placenta is functioning well, the baby's heart rate remains stable. If there is poor function of the placenta, the baby's heart rate will temporarily slow after a contraction. Looking at the baby's heart rate in response to uterine contractions can help the doctor estimate how the baby will respond to the stresses experienced during labor. This test is not performed as commonly as the NST or biophysical profile.

Although amniocentesis (removal of a sample of amniotic fluid with a long, thin needle) is most often done during the second trimester, there are certain conditions that may warrant an amniocentesis later in pregnancy. These conditions include a suspicion of chorioamnionitis or a risk of premature delivery, since amniocentesis fluid can be used to estimate maturity of the fetal lungs.

Ultrasound examinations may be performed in the third trimester if needed to help evaluate fetal growth and look for problems with the placenta.