Why Is Gestational Diabetes Screening Done at 24–28 Weeks?
What the Timing of Blood Sugar Testing Tells Us About Your Pregnancy
If you’re pregnant, you may be told that screening for gestational diabetes mellitus (GDM) usually happens between 24 and 28 weeks of pregnancy. Many parents wonder: Why that window? Why not earlier? And what does this test actually tell me about my body? Understanding the “why” behind the timing can help you feel more informed and empowered in your pregnancy care.
Pregnancy Naturally Changes How Your Body Handles Glucose
As pregnancy progresses, hormones produced by the placenta make the body more resistant to insulin, which means glucose stays in the bloodstream longer after meals. This is a normal physiologic adaptation designed to ensure that enough glucose is available for fetal growth. However, for some individuals, this insulin resistance becomes excessive, leading to elevated blood glucose levels and a diagnosis of gestational diabetes mellitus (Hillier et al., 2021).
The 24–28 week period is when this insulin resistance typically becomes clinically apparent. Screening during this window improves the ability to identify individuals whose glucose regulation is no longer adequate to meet the metabolic demands of pregnancy (Hillier et al., 2021).
What Happens If Blood Glucose Is Tested Earlier (e.g., at 20 Weeks)?
Screening earlier in pregnancy may fail to detect gestational diabetes because insulin resistance is often not yet pronounced. At 20 weeks, many individuals still demonstrate relatively normal glucose tolerance, even if they are destined to develop glucose intolerance later in pregnancy. As a result, early testing can lead to false reassurance, missing cases that only become evident as pregnancy-related insulin resistance increases (Hillier et al., 2021).
Emerging research using continuous glucose monitoring (CGM) suggests that subtle changes in glucose patterns may occur earlier in pregnancy, but these changes often do not meet diagnostic thresholds for gestational diabetes until later gestation (Quah et al., 2024). This reinforces why mid-pregnancy remains the most reliable screening window.
What This Timing Reveals About Glucose Processing in Early Pregnancy
In early pregnancy, most individuals can still compensate for rising insulin resistance by increasing insulin secretion. This means glucose levels may remain within normal ranges even as metabolic demands increase. Over time, however, the placenta produces higher levels of hormones that interfere with insulin action, making it more difficult for the body to maintain normal glucose control (Hillier et al., 2021).
By 24–28 weeks, this physiologic stress test of the pancreas reveals who can adapt metabolically—and who may benefit from additional support, monitoring, and interventions to protect both maternal and fetal health.
Why This Is Important for You to Know as a Patient
Understanding the timing of screening helps explain that gestational diabetes is not caused by personal failure, but rather reflects normal pregnancy physiology interacting with individual metabolic capacity. Even mild glucose intolerance has been associated with differences in metabolic factors and perinatal outcomes, underscoring the importance of timely detection (Scifres et al., 2024).
Once identified, gestational diabetes can be effectively managed. Research shows that real-time glucose monitoring and individualized management strategies can improve glycemic control and support both physical and mental well-being during pregnancy (Liu et al., 2025; Valent et al., 2025). Lifestyle strategies such as timing of physical activity around meals can also meaningfully improve post-meal glucose levels in those diagnosed with GDM (Brislane et al., 2023).
In addition, the use of continuous or flash glucose monitoring during pregnancy has been shown to enhance glucose awareness and improve glucose profiles, particularly in individuals struggling with glycemic control (Tumminia et al., 2021). Early feedback on glucose patterns may even influence future risk of developing gestational diabetes later in pregnancy (Quah et al., 2024).
Takeaway
The 24–28 week screening window is carefully chosen because it reflects the point in pregnancy when insulin resistance typically peaks and gestational diabetes becomes detectable. Testing too early may miss important changes in glucose metabolism, while timely screening allows for early intervention, education, and personalized support. Knowing this can help you approach testing with less fear and more understanding—seeing it as a routine check of how your body is adapting to the incredible metabolic work of pregnancy.
References
Brislane, Á., Reid, L. A., Bains, G., Greenwall, K., Khurana, R., & Davenport, M. H. (2023). Optimizing blood glucose control through the timing of exercise in pregnant individuals diagnosed with gestational diabetes mellitus. International Journal of Environmental Research and Public Health, 20(8), 5500. https://doi.org/10.3390/ijerph20085500
Hillier, T. A., Pedula, K. L., Ogasawara, K. K., Vesco, K. K., Oshiro, C. E. S., Lubarsky, S. L., & Van Marter, J. (2021). A pragmatic, randomized clinical trial of gestational diabetes screening. The New England Journal of Medicine, 384(10), 895–904. https://doi.org/10.1056/NEJMoa2026028
Liu, M., Chen, T., Wang, S., Li, N., & Liu, D. (2025). To assess the impact of individualized strategy and continuous glucose monitoring on glycemic control and mental health in pregnant women with diabetes. Frontiers in Endocrinology, 16, 1470473. https://doi.org/10.3389/fendo.2025.1470473
Quah, P. L., Tan, L. K., Lek, N., Tagore, S., Chern, B. S. M., Ang, S. B., Wright, A., Thain, S. P. T., & Tan, K. H. (2024). Continuous glucose monitoring feedback in the subsequent development of gestational diabetes: A pilot, randomized, controlled trial in pregnant women. American Journal of Perinatology, 41(Suppl 01), e3374–e3382. https://doi.org/10.1055/s-0043-1778664
Scifres, C. M., Davis, E. M., Orris, S., Costacou, T., Lalama, C., Abebe, K. Z., & Catalano, P. (2024). Metabolic factors and perinatal outcomes among pregnant individuals with mild glucose intolerance. Diabetes Research and Clinical Practice, 216, 111830. https://doi.org/10.1016/j.diabres.2024.111830
Tumminia, A., Milluzzo, A., Festa, C., Fresa, R., Pintaudi, B., Scavini, M., Vitacolonna, E., Napoli, A., & Sciacca, L. (2021). Efficacy of flash glucose monitoring in pregnant women with poorly controlled pregestational diabetes (FlashMom): A randomized pilot study. Nutrition, Metabolism and Cardiovascular Diseases, 31(6), 1851–1859. https://doi.org/10.1016/j.numecd.2021.03.013
Valent, A. M., Rickert, M., Pagan, C. H., Ward, L., Dunn, E., & Rincon, M. (2025). Real-time continuous glucose monitoring in pregnancies with gestational diabetes mellitus: A randomized controlled trial. Diabetes Care, 48(9), 1581–1588. https://doi.org/10.2337/dc25-0115
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