Third Trimester Screening
During the 3rd trimester, you'll likely have these tests:
Glucose testing. Sometime between weeks 24 and 28, your doctor will test to see if your body is using sugar correctly. You'll drink a very sugary liquid and then, an hour later, you'll have a blood test. You do not need to fast before this 1-hour glucose test, but don't have a doughnut on your way to the doctor's office either. If there is excess sugar in your blood, your doctor will order additional bloodwork to determine whether you have gestational diabetes.
Hematocrit. This blood test determines whether you have anemia, a shortage of iron in the blood.
Syphilis screen. A woman can have syphilis without knowing it, and she can unknowingly pass the infection on to her baby. If she tests positive for the bacteria, she can be treated with antibiotics during pregnancy, and her baby will receive antibiotic treatment shortly after birth. Without treatment, some babies who are infected in the uterus die within a few days of delivery. Those who survive are at very high risk for blindness, brain damage, hearing loss, and problems with bones, skin, and teeth.
Rh factor. Your blood was checked for Rh factor at your first prenatal visit. (Rh factor is a protein that is found on red blood cells. Most women have this protein, but about 15 percent of Caucasian women and 7 percent of African-American women don't. They are considered to be "Rh-negative.") Many doctors will repeat the Rh test sometime in weeks 26-28 to confirm your Rh status, particularly if you tested Rh-negative earlier in your pregnancy and if the baby's father is Rh-positive. In such cases, there is a chance your baby will have Rh-positive blood and your body will build up antibodies to it during pregnancy. If your body does develop these antibodies, they could jeopardize future pregnancies.
If you are Rh-negative, you will get a shot of Rhogam (Rh-immune globulin) at 26-28 weeks as a precaution; it fights those antibodies if a small amount of Rh-positive fetal blood has mixed with your blood.
If you are Rh-negative and your baby is Rh-positive, you will receive an injection of Rhogam (RhIg) within 72 hours of delivery to help prevent an immune response against Rh-positive blood. If your baby turns out to be Rh-negative, then you will not need this injection.
Originally published in You & Your Baby: Pregnancy.
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