Prenatal Visits in the Third Trimester

By Joanne Stone, Keith Eddleman, Mary Duenwald

Between 28 and 36 weeks, your practitioner probably wants to see you every two to three weeks, and then weekly as you close in on delivery. She takes the usual measurements: blood pressure, weight, fetal heart rate, fundal height, and urine tests. These visits are a good time to discuss issues related to labor and delivery with your practitioner.

If you don’t deliver by your due date, your practitioner may want to start performing non-stress tests. These tests assess fetal well-being. After 40 to 41 weeks, placental function and amniotic fluid may decline, and ensuring that both remain adequate to support the pregnancy is important. By 42 weeks, many practitioners recommend inducing labor because the risk of problems for the baby rises significantly after that time.

As your pregnancy winds down, your practitioner may perform certain tests to make sure that your baby is as healthy as possible.

Taking Group B strep cultures

The only routine test that may be performed during one of your final prenatal visits is a culture for Group B strep, bacteria commonly found in the vagina and rectum. The Centers for Disease Control and Prevention (CDC) and the American College of Obstetricians and Gynecologists now recommend that all women be routinely screened for Group B strep at around 36 weeks gestation.

About 15 to 20 percent of women harbor this organism. If the culture is positive at 36 weeks, your doctor will recommend that you receive antibiotics during labor to reduce the risk of transmitting the bacteria to the baby. Treating the bacteria any earlier doesn’t help, because it can come back by the time you’re in labor.

Gauging lung maturity

If you’re planning a repeat cesarean delivery (meaning that you had one in an earlier pregnancy) or an elective induction at less than 39 weeks, some practitioners may recommend that you have an amniocentesis to establish that the fetus’s lungs are mature and ready to function.

The most commonly performed test for lung maturity is called L/S ratio, which measures the ratio of lecithin to sphingomyelin (both are substances found in the amniotic fluid). If the L/S ratio is 2.0 or greater or if PG (phosphatidyl glycerol, a substance produced by mature lung cells) is present in the amniotic fluid, the baby’s lungs are considered mature.

Assessing your baby’s current health

At certain times, your practitioner may suggest that you undergo tests for the baby. These tests, also referred to as antepartum fetal surveillance, check the baby’s well-being. Your practitioner can perform these tests at any time after about 24 to 26 weeks if cause for concern exists, or after 41 weeks if you haven’t delivered. Several different tests can be used.

Non-stress test (NST)

Non-stress testing consists of measuring the fetal heart rate, fetal movement, and uterine activity using a special monitoring machine. Your practitioner hooks you up to this device, which picks up uterine contractions and the baby’s heart rate and generates a tracing of both. The NST is similar to the device used during labor to monitor the fetal heart rate and contractions.

You also receive a button to press each time you perceive fetal movement. The monitoring goes on for about 20 to 40 minutes. The doctor then looks at the tracing for signs of accelerations, or increases, in the fetal heart rate.

If accelerations are present and occur often enough, the test is considered reactive, and the fetus is thought to be healthy and should continue to be so for three to seven days. (The fetus is healthy in more than 99 percent of cases.)

If the accelerations aren’t adequate (that is, the test is nonreactive), you still have no cause for alarm. In 80 percent of cases, the fetus is fine and probably just in sleep cycle, but further evaluation is needed.

Contraction stress test (CST)

The contraction stress test is similar to a non-stress test except that the fetal heart is timed in relation to uterine contractions. The contractions sometimes occur by themselves, but more often are brought on with low doses of oxytocin (Pitocin) or by nipple stimulation.

Don’t stimulate your nipples at home to bring on contractions. Perform nipple stimulation only under your doctor’s supervision, because you doctor wants to monitor you and make sure that the uterus doesn’t contract too much.

Three good contractions in a ten-minute period need to be present in order for the test to be interpreted. If the fetal heart rate doesn’t drop after the contractions, the test is considered negative, and the baby is thought to be fine for at least one more week.

If the test is positive (the fetal heart rate does drop after the contractions) or suspicious, your practitioner investigates the situation further. Proper management depends on your particular situation. A CST is performed if the results of the non-stress test are inconclusive, or if your doctor wants additional testing of fetal well-being.

Biophysical profile (BPP)

A biophysical profile, which combines ultrasound with a non-stress test, may be performed instead of the NST alone, or in addition to the NST if further testing is warranted. Which test is performed (NST or BPP) is often just a matter of physician preference.

The BPP evaluates the following:

  • Fetal movements, observed by ultrasound

  • Fetal body tone, observed by ultrasound

  • Fetal breathing movements for 30 seconds in a row (chest motions that mimic breathing), observed by ultrasound

  • Quantity of amniotic fluid, observed by ultrasound

  • Non-stress test

Vibracoustic stimulation

Your doctor may perform a vibracoustic stimulation test during a non-stress test. During the test, the fetus’s response to stimulation by sound or vibrations is observed. The practitioner “buzzes” the mother’s belly with a vibrating device, which causes a transmission of sound or vibrations to the fetus. Normally, the fetal heart rate accelerates when the fetus is stimulated in this way.

Doppler velocimetry

A doctor performs a Doppler velocimetry test only in certain situations — if certain fetal problems exist, for example, or if you have high blood pressure. Basically, with this test, your doctor performs a special type of ultrasound exam that assesses the blood flow through the umbilical cord.