From the Guidelines
During the second and third trimesters of pregnancy, additional laboratory tests are drawn to monitor maternal and fetal health, including a glucose screening test, complete blood count (CBC), Group B Streptococcus (GBS) screening, and repeat antibody screening for Rh-negative women. These tests are crucial for identifying potential complications that may arise during later pregnancy.
- A glucose screening test is performed between 24-28 weeks to check for gestational diabetes, as recommended by the American College of Obstetricians and Gynecologists (ACOG) and supported by standards of medical care in diabetes 1.
- A complete blood count (CBC) is repeated to monitor for anemia, which is common as pregnancy progresses due to increased blood volume.
- Group B Streptococcus (GBS) screening is performed between 35-37 weeks via a vaginal and rectal swab to identify women who need antibiotics during labor to prevent transmission to the newborn.
- Repeat antibody screening for Rh-negative women is performed to determine if they require RhoGAM at 28 weeks if not sensitized.
- Some providers also recheck thyroid function tests in women with thyroid disorders, as pregnancy can affect thyroid hormone requirements.
- Urinalysis may be repeated to screen for protein (preeclampsia) or infection. Additionally, HIV screening is recommended for all pregnant women, with a second test considered in the third trimester for women at high risk of acquiring HIV infection 1. The most recent and highest quality study supports the importance of these tests in monitoring maternal and fetal health during the second and third trimesters of pregnancy.
From the Research
Additional Laboratory Tests in Pregnancy
During the second and third trimesters of pregnancy, several additional laboratory tests are drawn to monitor the health of the mother and the fetus. These tests include:
- Glucose screening tests to detect gestational diabetes mellitus (GDM) 2, 3, 4
- Glycosylated hemoglobin measurements to assess glucose control 2, 5
- Fasting plasma glucose measurements to predict macrosomia 5
- Glycosylated plasma protein measurements to predict macrosomia 5
- Urine tests for glycosuria, although the sensitivity and positive predictive value of this test are low 3
Timing of Laboratory Tests
These laboratory tests are typically drawn between 24 and 28 weeks of gestation, although some studies suggest that glucose intolerance can occur in the early weeks of gestation and that repeat testing may be necessary 4. The American College of Obstetricians and Gynecologists recommends screening for GDM at 24 to 28 weeks of gestation, but some studies suggest that screening may be necessary earlier in pregnancy, especially in high-risk patients 6.
Risk Factors for Gestational Diabetes
Women with risk factors for gestational diabetes, such as a family history of diabetes, obesity, or a previous history of gestational diabetes, may require earlier and more frequent screening 6. A thorough history should be taken to determine whether a woman has risk factors for gestational diabetes, and screening should be tailored accordingly.