Third Trimester Prenatal Tests

You may be offered several prenatal tests throughout your pregnancy. Some tests are performed routinely on all pregnant women, such as blood tests to determine blood type and iron levels. Some tests are done to screen women for a condition they or their baby are at risk for developing. Other tests, such as Chorionic Villus Sampling (CVS) and amniocentesis, are considerations for women 35 years of age and older or those who may be at risk of delivering a baby with certain birth defects. Some tests are done later in pregnancy to measure the baby’s well-being and to make sure that the baby is growing and developing at the expected rate.

The following is a brief description of some of the more common prenatal tests. Your healthcare provider, and sometimes a genetic counselor, can help you determine which tests might be recommended for your pregnancy and help you when making decisions.

Nonstress Test (NST)

A NST helps determine your baby’s well-being. An electronic fetal monitor is used to trace your baby’s heart rate to see if it speeds up or slows down as the baby moves.

Contraction Stress Test (CST)

A CST helps determine how well your baby will handle the stress of labor. During a CST, your nipples are stimulated to release oxytocin which causes your uterus to contract. An electronic fetal monitor is used to trace your baby’s heart rate and look for changes in relation to your uterine contractions.

Oxytocin Challenge Test (OCT)

An OCT is the same as a CST except contractions are brought on by a small dose of pitocin, a synthetic form of the hormone oxytocin, administered through a vein in your arm. An OCT is usually done if the contractions from nipple stimulation are not strong enough to assess how your baby will handle the stress of labor.

Biophysical Profile (BPP)

A BPP is a test of your baby’s well-being and may be done later in your pregnancy. Abdominal ultrasound is used to assess your baby’s heartbeat, movement and breathing pattern, along with the placenta and amount of amniotic fluid. BPP is considered an “in utero” check-up of your baby.

Group B Streptococcal Disease (GBS)

GBS is a bacteria that is naturally present in the gastrointestinal (GI) tract. Sometimes in pregnancy, GBS may get into and grow in the rectal area or vagina (1 in 7 women). If GBS is not detected and treated with antibiotics, it may transfer to the baby and place the baby at risk for serious infection (1 per 150 births) or even death (1 per 1,000 births).

To test if GBS is growing in your vagina or rectum, your healthcare provider should culture these areas between 35 and 37 weeks of pregnancy according to the most recent recommendations. If GBS is present, antibiotics can be given during labor, which greatly reduces the above mentioned complications.