Gestational diabetes mellitus (GDM) is a condition defined by glucose intolerance first recognized during pregnancy. This metabolic disorder arises when the body cannot produce or properly use the insulin needed to regulate blood sugar levels, which typically increase due to hormonal changes. Screening for GDM is a standard part of prenatal care today because elevated maternal glucose levels can lead to significant complications for both the mother and the developing fetus. Identifying and managing this condition is a public health priority to prevent adverse outcomes.
Defining Gestational Diabetes Before Formal Testing
The medical community recognized the connection between high blood sugar in pregnancy and poor birth outcomes long before standardized testing existed. Case reports dating back to the 19th century documented severe complications, such as the delivery of excessively large babies (fetal macrosomia), in mothers with overt diabetes. Diagnosis in the early 20th century was largely retrospective, based on severe symptoms like excessive thirst or the finding of sugar in the urine (glycosuria).
The term Gestational Diabetes Mellitus gained acceptance in the 1950s, along with the identification of certain risk factors. Without a formal screening protocol, however, women with milder glucose intolerance often went undiagnosed. This lack of systematic testing meant that many pregnancies affected by GDM resulted in high rates of adverse events, including stillbirth and perinatal mortality. This established a clear need for a reliable method of early detection.
The Pioneering Era of Screening
The transition from recognizing the problem to systematically testing for it began in the 1960s. The formal start of systematic testing is credited to the foundational research of Dr. John O’Sullivan and Dr. Claire Mahan, working in Boston. In 1964, they published criteria for the Oral Glucose Tolerance Test (OGTT) based on a study of 752 pregnant women.
Their work established the first statistically based criteria for assessing glucose tolerance in pregnancy, initially aiming to predict a mother’s future risk of developing type 2 diabetes. The O’Sullivan-Mahan criteria involved using a 100-gram glucose load and measuring blood glucose levels multiple times over a three-hour period. This research ultimately formed the basis of the “two-step” screening approach that would be widely adopted for decades.
The two-step method involved a non-fasting 50-gram glucose challenge test (GCT) as a preliminary screen for all pregnant women. If the one-hour blood glucose value exceeded a specific threshold, the woman was recalled for the more time-consuming, diagnostic 100-gram, three-hour OGTT. This two-step process allowed for the standardized mass screening of pregnant populations.
Standardization and Shifting Diagnostic Thresholds
Following the initial adoption of the O’Sullivan criteria, the medical community faced the challenge of standardizing the diagnostic thresholds. A major change occurred when laboratory analysis methods shifted from using whole blood to plasma for glucose measurements, requiring an adjustment of the original O’Sullivan-Mahan values. In 1979, the National Diabetes Data Group (NDDG) proposed new criteria, raising the cut-off values to account for this change in laboratory technique.
The NDDG criteria became a widely used standard, but debate continued over whether the original criteria were strict enough to identify all women at risk for adverse perinatal outcomes. This led to the introduction of the Carpenter/Coustan criteria in the late 1980s and early 1990s, which lowered the plasma glucose thresholds compared to the NDDG values.
The Carpenter/Coustan thresholds were more inclusive, diagnosing a greater number of women with GDM, typically resulting in an increase in prevalence by about 50%. Studies demonstrated that women diagnosed only by the Carpenter/Coustan criteria—and not the looser NDDG criteria—still had an increased risk for complications like fetal macrosomia and pre-eclampsia. This finding provided the clinical justification for adopting the stricter criteria, as it ensured that more women who could benefit from treatment were identified.
Modern Screening Protocols
The most significant recent change to global standards came in 2010 with the release of the International Association of Diabetes and Pregnancy Study Groups (IADPSG) criteria. These recommendations were based on the large-scale Hyperglycemia and Adverse Pregnancy Outcomes (HAPO) study, which demonstrated that maternal and fetal risks increased continuously across the range of glucose levels below the older diagnostic thresholds. The IADPSG introduced a “one-step” approach, using a single 75-gram, two-hour OGTT for diagnosis, and only requiring one elevated value to confirm GDM.
The adoption of the IADPSG criteria resulted in a substantial increase in the prevalence of GDM, sometimes doubling or tripling the number of women diagnosed compared to the older criteria. Despite endorsement by the World Health Organization (WHO), the IADPSG approach remains controversial in some regions, including parts of the United States. Many major organizations continue to recommend the traditional two-step approach utilizing the Carpenter/Coustan thresholds, citing concerns about the increased cost and logistical strain of screening many more women.