Working at Northwestern Medicine Kishwaukee Hospital, I see firsthand how the third trimester is an exciting time for expecting mothers. The last few weeks of pregnancy mark a time of preparation for the baby’s arrival. This preparation includes a number of prenatal visits with your obstetrician.
According to the American College of Obstetricians and Gynecologists (ACOG), the third trimester starts at 28 weeks 0 days and lasts until 40 weeks 6 days. During this time, patients will receive a variety of tests to ensure the final stage of pregnancy is going smoothly up until the point of delivery.
Prenatal visits
Between 28 to 36 weeks, patients come for prenatal visits every two weeks. Between 36 weeks until delivery, patients come in weekly for prenatal visits. During these visits, a patient’s blood pressure is checked. Sometimes a urine test is also done. The purpose of these tests is to monitor for gestational hypertension (high blood pressure caused by the pregnancy) and preeclampsia.
Preeclampsia is a blood pressure condition that most commonly manifests in the third trimester. Symptoms include high blood pressure and large amounts of protein in the urine. Women may also develop headaches and blurry vision from preeclampsia.
If preeclampsia develops before 37 weeks, expecting mothers will be monitored closely to hopefully avoid early delivery. Some patients are asked to monitor their blood pressure at home, or patients may be admitted to the hospital until delivery. If preeclampsia develops after 37 weeks, the baby will likely be delivered early. Pregnant people can have a natural birth with preeclampsia, though sometimes a cesarean section (C-section) is recommended by care teams.
Diabetes screening test
A diabetes screening test is common at the end of the second trimester or beginning of the third trimester, between 24 to 28 weeks of gestation, according to ACOG. Patients at high risk for diabetes are tested in the first trimester.
The test consists of drinking a beverage with 50 grams of sugar in it. Ideally, the beverage should be consumed within 5 minutes. The patient’s blood sugar is then tested 1 hour later. If a patient has an abnormally high glucose level on the 1-hour glucose test, then the patient is instructed to do a 3-hour glucose tolerance test.
During a 3-hour glucose tolerance test, patients will be asked to fast for 8 hours before the test. The beverage for the 3-hour test has 100 grams of sugar. A patient’s fasting blood glucose is checked prior to consuming the beverage. It’s then checked one hour after consuming the beverage, at the 2-hour mark and finally at the 3-hour mark. If a patient fails two out of the four blood sugar checks, they’re diagnosed with gestational diabetes.
HIV test and syphilis screening
Although women are screened in the first trimester, an HIV test is repeated in the third trimester to ensure the pregnant person hasn’t become infected since the first trimester test. If a pregnant person tests positive, it allows doctors to get the patient appropriate treatment to minimize transmission to the fetus during delivery.
Much like the HIV test, pregnant people are screened for syphilis, a sexually transmitted infection, in the first and third trimesters. Syphilis is typically transmitted via unprotected sex with another individual who is infected. If a pregnant person is positive, it’s important to treat the infection to minimize risks to the fetus.
Both HIV and syphilis can have a profoundly negative impact on the baby if not diagnosed prior to delivery.
Other tests
Complete blood count (CBC)
A complete blood count (CBC) checks for anemia, which is common in the third trimester of pregnancy due to the changes the body goes through during pregnancy. This also checks to ensure that a pregnant person’s platelet level is normal and they haven’t developed any pregnancy complications, such as gestational thrombocytopenia (abnormally low platelets).
Ultrasound
In high-risk pregnancies, an ultrasound to check the growth of the baby may be done in the third trimester, usually around 30 to 32 weeks. Based on the results, subsequent ultrasounds may also be done.
Weekly antenatal testing
In high-risk pregnancies, pregnant people may undergo weekly antenatal testing that consists of monitoring the heartbeat of the fetus for 20 to 30 minutes, known as a non-stress test (NST), as well as an ultrasound to measure the amount of amniotic fluid.
Group B streptococcus testing (GBS)
A group B streptococcus (GBS) test is performed between 36 to 38 weeks of pregnancy. For this test, the vaginal and rectal areas are swabbed with a soft cotton swab for bacteria. If the test is positive, pregnant people are treated when they go into labor.
GBS is part of the normal vaginal flora. It’s not a sexually transmitted infection (STI). It doesn’t cause harm to the pregnant person, but if the fetus comes through the birth canal of a person with GBS, it may cause an infection. According to Nemours Children’s Health, about 1 in 4 pregnant people carry GBS. About 1 to 2 babies out of 100 will become infected with GBS bacteria if the pregnant person isn’t treated during labor, according to the March of Dimes. The chance of a newborn becoming sick is much lower when the pregnant person receives treatment.
Treatment for GBS during labor includes giving the pregnant person antibiotics via an intravenous line (IV) during labor. Antibiotics should be given for at least four hours before delivery.
Fetal movement
Patients should carefully monitor fetal movement during the third trimester. There are many different ways to do fetal kick counts at home. One common recommendation is to feel for 10 kicks within two hours. Patients should try to do the kick counts at the same time each day. If there’s a decrease in fetal movement, patients should immediately notify their healthcare provider.
