Routine Screening at 24 Weeks of Pregnancy
At 24 weeks of pregnancy, the routine screening test is the Gestational Diabetes Mellitus (GDM) screen. 1, 2
GDM Screening at 24 Weeks
The USPSTF strongly recommends screening for GDM in all asymptomatic pregnant women after 24 weeks of gestation (B recommendation). 1 This timing represents the standard window for universal GDM screening, which extends from 24-28 weeks of gestation. 2, 3
Screening Methods
The most common approach in the United States is the 2-step method: a 50-g oral glucose challenge test (OGCT) in a non-fasting state, followed by a 100-g oral glucose tolerance test (OGTT) if the initial screening threshold is met or exceeded. 2, 4
The alternative 1-step approach uses a 75-g OGTT after an overnight fast of at least 8 hours, with diagnostic thresholds of fasting ≥92 mg/dL, 1-hour ≥180 mg/dL, and 2-hour ≥153 mg/dL. 3, 4
Clinical Rationale
Early detection and treatment of GDM significantly reduces the risk for preeclampsia, fetal macrosomia, and shoulder dystocia. 1, 2
Women with GDM are at increased risk for maternal and infant complications, and screening provides an opportunity to prevent adverse outcomes through dietary modifications, glucose monitoring, and medication if needed. 1, 2
Other Screening Tests: Timing Considerations
Group B Streptococcus (GBS) Screening
- GBS screening is NOT performed at 24 weeks. Standard practice is to screen for GBS between 36-37 weeks of gestation, not at 24 weeks.
Maternal Serum Screening
- Maternal serum screening is NOT performed at 24 weeks. This screening (for chromosomal abnormalities and neural tube defects) is typically performed in the first trimester (10-13 weeks) or early second trimester (15-20 weeks), well before 24 weeks.
STI Screening
- STI screening is NOT routinely performed at 24 weeks. Initial STI screening (including HIV, syphilis, hepatitis B, chlamydia, and gonorrhea) is performed at the first prenatal visit in the first trimester. Repeat testing may be indicated in the third trimester for high-risk populations, but 24 weeks is not a standard timing for routine STI screening.
Important Clinical Considerations
Women at very high risk (BMI ≥30 kg/m², prior GDM, family history of diabetes, high-risk ethnicity) should have already undergone early screening at their first prenatal visit (12-14 weeks) and require rescreening at 24-28 weeks if initially negative. 3, 4
The 50-g OGCT has variable sensitivity and specificity depending on the threshold used (typically 130-140 mg/dL), which should be considered when interpreting results. 2, 4
False positive results can cause unnecessary anxiety, highlighting the need for confirmatory testing with the 100-g OGTT before making a definitive diagnosis. 2