Introduction

Gestational diabetes mellitus (GDM) is defined as glucose intolerance that is first detected during pregnancy, typically in the second or third trimester, and that is not clearly overt diabetes prior to gestation. It is one of the most common medical complications of pregnancy, affecting approximately 6–9% of pregnancies in the United States and up to 14% in some populations globally. Timely and accurate screening is essential because poorly controlled GDM is associated with increased risks of preeclampsia, cesarean delivery, neonatal hypoglycemia, macrosomia, and long-term metabolic disorders for both mother and child. Screening recommendations vary by trimester due to differences in physiological changes and risk stratification. This article reviews current guidelines from major organizations—including the American College of Obstetricians and Gynecologists (ACOG), the American Diabetes Association (ADA), the World Health Organization (WHO), and the International Association of Diabetes and Pregnancy Study Groups (IADPSG)—and provides a practical framework for clinicians conducting GDM screening across all trimesters.

First‑Trimester Screening

Rationale and Target Population

Screening for glucose disorders in the first trimester is not performed universally. Instead, it is reserved for women with high‑risk characteristics because the primary goal is to detect pre‑existing diabetes (including type 2 diabetes) or very early‑onset GDM. Identifying overt diabetes early allows for immediate management changes and reduces the risk of congenital anomalies and miscarriage associated with hyperglycemia during organogenesis.

The ADA and ACOG recommend early screening for women with one or more of the following risk factors:

  • Body mass index (BMI) ≥ 30 kg/m² (or ≥ 25 kg/m² in some Asian populations)
  • Prior history of GDM
  • First‑degree relative with diabetes mellitus
  • History of polycystic ovary syndrome (PCOS) or metabolic syndrome
  • Glycosuria on routine urine dipstick
  • Previous delivery of a macrosomic infant (≥ 4,000 g) or history of stillbirth of unknown etiology
  • High‑risk ethnic background (e.g., Hispanic, African American, Native American, South Asian, or Pacific Islander)
  • Age ≥ 35 years

First‑trimester screening typically uses one of several approaches:

  • Fasting plasma glucose (FPG): A fasting value ≥ 7.0 mmol/L (126 mg/dL) is diagnostic of overt diabetes; values between 5.1 and 6.9 mmol/L (92–125 mg/dL) raise suspicion and often lead to an oral glucose tolerance test (OGTT).
  • Hemoglobin A1c (HbA1c): A value ≥ 6.5% (48 mmol/mol) confirms pre‑existing diabetes; values 5.7–6.4% (39–47 mmol/mol) indicate increased risk.
  • Early 75‑gram OGTT: Some guidelines (e.g., IADPSG) suggest performing a full 2‑hour OGTT in high‑risk women during the first visit. FPG ≥ 5.1 mmol/L or 1‑hour ≥ 10.0 mmol/L or 2‑hour ≥ 8.5 mmol/L at that point is consistent with GDM.

It is important to note that using Carpenter‑Coustan thresholds for a 3‑hour 100‑g OGTT is also practiced in some settings, but the 75‑g test is increasingly preferred for its simplicity and international consensus. If early screening is negative, universal rescreening is performed at 24–28 weeks.

External link: ACOG Practice Bulletin on GDM

Second‑Trimester Screening

Universal Screening Window: 24–28 Weeks

The second trimester is the most critical period for GDM detection because the physiological insulin resistance that normally occurs during pregnancy peaks around 24–28 weeks, driven by placental hormones such as human placental lactogen, progesterone, and growth hormone. Nearly 90% of GDM cases are identified during this window.

Both ACOG and ADA recommend that all pregnant women not known to have pre‑existing diabetes undergo screening at 24–28 weeks. The two principal approaches are:

  • Two‑step approach (ACOG recommendation): A non‑fasting 50‑g glucose challenge test (GCT). If the 1‑hour plasma glucose is ≥ 7.8 mmol/L (140 mg/dL), proceed to a diagnostic 100‑g 3‑hour OGTT using Carpenter‑Coustan criteria (two or more elevated values needed for diagnosis: fasting ≥ 5.3 mmol/L, 1‑hour ≥ 10.0 mmol/L, 2‑hour ≥ 8.6 mmol/L, 3‑hour ≥ 7.8 mmol/L).
  • One‑step approach (ADA/IADPSG recommendation): A single 75‑g 2‑hour OGTT after overnight fast. Diagnostic thresholds: fasting ≥ 5.1 mmol/L, 1‑hour ≥ 10.0 mmol/L, or 2‑hour ≥ 8.5 mmol/L. Any one value is sufficient for diagnosis.

The choice between one‑step and two‑step methodologies remains controversial, primarily because the one‑step approach identifies more cases (leading to increased resource utilization and potential overdiagnosis) with uncertain evidence of improved outcomes. ACOG continues to endorse the two‑step method as the standard in the United States, while many international guidelines (including WHO and IADPSG) have adopted the one‑step approach.

Clinical Implications of Second‑Trimester Diagnosis

When GDM is diagnosed at 24–28 weeks, management typically includes medical nutrition therapy, glucose self‑monitoring, and possibly pharmacotherapy (insulin or metformin). Aggressive treatment has been shown to reduce the incidence of macrosomia, shoulder dystocia, and neonatal hypoglycemia in large randomized trials (e.g., the HAPO study and the ACHOIS trial).

External link: American Diabetes Association – Gestational Diabetes

Third‑Trimester Screening

When Rescreening Is Indicated

Routine rescreening in the third trimester is not recommended for most women who have already tested negative at 24–28 weeks. However, certain scenarios warrant additional testing after 28 weeks:

  • Women who presented late for prenatal care and missed the standard screening window.
  • Women who develop new risk factors (e.g., significant weight gain, polyhydramnios, or clinical signs of fetal macrosomia).
  • Women with borderline negative results earlier (e.g., GCT 7.5–7.7 mmol/L ) who are at increased risk for developing GDM later.
  • In some high‑risk populations, a second OGTT at 32–34 weeks may be considered, though evidence is limited.

Screening in the third trimester uses the same diagnostic tests as the second trimester. Clinicians should be aware that as pregnancy advances, maternal glucose metabolism continues to change; a woman who had a normal result at 26 weeks may develop hyperglycemia by 32 weeks. For this reason, many experts advocate “universal late screening” in resource‑rich settings, but no major guideline currently mandates it.

External link: A review of third‑trimester GDM screening (PubMed Central)

Postpartum Screening and Long‑Term Follow‑Up

Why Postpartum Testing Matters

Women with GDM have a 5–10 fold increased risk of developing type 2 diabetes within 5–10 years after delivery. Furthermore, many who had GDM actually had undiagnosed pre‑existing type 2 diabetes that was unmasked by pregnancy. Therefore, postpartum screening is mandatory for all women with any history of GDM.

  • Six‑ to twelve‑week postpartum: A 75‑g 2‑hour OGTT is the preferred test because it detects both impaired glucose tolerance and type 2 diabetes. Fasting glucose alone may miss up to 30% of cases. New mothers should breastfeed before the test if possible.
  • After the postpartum period: The ADA recommends lifelong screening every 1–3 years, depending on additional risk factors. Those with prediabetes (FPG 5.6–6.9 mmol/L or HbA1c 5.7–6.4%) should be tested annually and receive lifestyle interventions.

Many women fail to attend postpartum glucose testing due to competing demands of infant care. Clinicians should schedule the test at the postpartum visit, provide clear instructions, and consider point‑of‑care HbA1c if OGTT is impractical. A recent study found that only about 50% of women with GDM receive any postpartum glucose testing, highlighting a significant gap in care.

External link: CDC – Gestational Diabetes and Type 2 Diabetes Prevention

Summary of Recommendations by Trimester

Key Takeaways for Clinicians

  • First trimester: Screen only high‑risk women using FPG, HbA1c, or early OGTT. Identify overt diabetes early to guide management.
  • Second trimester (24–28 weeks): Universal screening for all pregnant women. Two‑step (ACOG) or one‑step (ADA/WHO) approaches are acceptable based on local guidelines. Use the 75‑g OGTT criteria (one abnormal value) for the one‑step method or the 100‑g 3‑hour OGTT for the two‑step method.
  • Third trimester: Consider rescreening only for those who missed earlier testing or develop new risk factors. Routine late screening is not standard but may be reasonable in high‑risk populations.
  • Postpartum and beyond: All women with GDM need a 75‑g OGTT at 6–12 weeks postpartum, followed by lifelong periodic screening for type 2 diabetes.

Adhering to these trimester‑specific recommendations enables early detection of hyperglycemia, reduces adverse pregnancy outcomes, and offers an opportunity to prevent or delay type 2 diabetes in a high‑risk population. Clinicians should remain flexible and apply shared decision‑making, particularly when local resources and patient preferences influence test selection. For the most current guidance, refer to the International Diabetes Federation guidelines on GDM.