Published: September 17, 2026
Gestational diabetes affects roughly 12% of pregnancies and accounts for about 6 in 7 cases of diabetes during pregnancy, making a clear understanding of gestational diabetes diagnosis criteria essential for every primary care and obstetrics provider. Getting the timing and the testing sequence right informs outcomes for both mother and baby.
In a recent episode of Norton Healthcare’s “MedChat” podcast, a conversation between host Kris E. Barnsfather, M.D., OB/GYN with Norton Women’s Care, and Kristine Y. Lain, M.D., a maternal-fetal medicine specialist and medical director for Norton Children’s Maternal-Fetal Medicine the Wendy Novak Diabetes Institute Perinatal program, discuss guidance for clinicians on diagnosing gestational diabetes.
Diagnosing gestational diabetes typically follows a two-step approach. The first step is a one-hour, 50-gram glucose challenge test, usually performed between 24 and 28 weeks of pregnancy. A plasma glucose result above 130 mg/dL to 140 mg/dL (the exact cutoff varies by community standard) triggers the second step: a fasting, three-hour glucose tolerance test using a 100-gram load.
A diagnosis is confirmed on the three-hour test when at least two of the four measured values — fasting, one hour, two hours or three hours — are elevated, or when the fasting value alone exceeds 126 mg/dL. A one-hour screening result above 200 mg/dL is diagnostic on its own, without need for the follow-up test.
Even a single abnormal value on the three-hour test has been linked to a higher risk of adverse outcomes, including miscarriage and congenital malformations. Patients with one abnormal value should be considered for glycemic monitoring or a repeat three-hour test within four weeks.
American Diabetes Association’s 2026 Standards of Care in Diabetes recommends: Use hemoglobin A1C to screen high-risk patients in the first trimester, ahead of the traditional 24- to 28-week window.
In the podcast, Dr. Lain pointed to a shift reflected in the American Diabetes Association’s 2026 Standards of Care in Diabetes: using hemoglobin A1C to screen high-risk patients in the first trimester, ahead of the traditional 24- to 28-week window.
A1C is most reliable early in pregnancy, before red blood cell turnover accelerates and reduces its accuracy later. In practice:
“A first-trimester A1C is not a replacement for the standard glucose tolerance test,” Dr. Lain said. “The diagnostic cutoffs were derived from midpregnancy glucose values and don’t translate perfectly to early pregnancy — but it remains the best available early predictor for identifying at-risk patients sooner. Practices should settle on one universal screening protocol for this population and apply it consistently.”
Diagnosis criteria aside, when elevated glucose occurs during pregnancy matters clinically. Hyperglycemia earlier in pregnancy — during organ formation — raises the risk of cardiac and neural tube anomalies and miscarriage. Hyperglycemia later in pregnancy is more closely tied to fetal growth complications: macrosomia, shoulder dystocia, neonatal hypoglycemia and neonatal intensive care unit admission. Poorly controlled diabetes overall increases the risk of congenital anomalies threefold to eightfold, with a 22% malformation risk for patients whose A1C exceeds 8.5.
Insulin remains the standard of care for gestational diabetes, since it does not cross the placenta while still lowering maternal blood glucose. Long-acting basal insulin and short-acting mealtime insulin are typically combined based on each patient’s glucose pattern — continuous glucose monitoring has made it easier to tailor regimens rather than react to single readings.
Oral agents are more nuanced. Metformin has a long track record in pregnancy and often is continued for patients who enter pregnancy already using it. Sulfonylureas, by contrast, cross the placenta, carry a higher reported failure rate for glycemic control and have largely fallen out of favor; patients on a sulfonylurea who are planning pregnancy should discuss transitioning to insulin or metformin.
With treatment, outcomes improve substantially: The risk of macrosomia drops from roughly 22% to 13%, and the risk of preeclampsia falls from about 18% to 12%.
Delivery timing is individualized based on glucose control, fetal growth and maternal status, but providers generally aim for 37 to 39 weeks, often recommending the earlier end of that range for patients on medication. Insulin needs are typically reassessed the day before delivery and monitored closely into the postpartum period.
Gestational diabetes diagnosis carries a lasting implication: About 1/2 of women with gestational diabetes go on to develop Type 2 diabetes.
Postpartum, the diagnosis carries a lasting implication: about half of women with gestational diabetes go on to develop Type 2 diabetes. Guidelines call for a glucose assessment — a 75-gram oral glucose tolerance test, A1C or, at minimum, a fasting glucose — at six to 12 weeks postpartum, followed by rescreening every one to two years for life. Pregnancy functions as a metabolic stress test that can reveal risk long before it would otherwise surface.
The Wendy Novak Diabetes Institute Perinatal Program, the first of its kind in Louisville or Southern Indiana, provides specialized support for managing diabetes before, during and after pregnancy. Integrated within the Norton Children’s Perinatal Center, our program brings together a team with diabetes expertise, specializing in complex pregnancies. This includes maternal-fetal medicine specialists, diabetes educators and clinical dietitians, working collaboratively to develop a personalized care plan for each patient to ensure the best possible outcomes for mom and baby. Collaboration with referring obstetricians, primary care providers and endocrinologists is a key component to the program. Our services include pre-pregnancy counseling, pregnancy monitoring and postpartum care, providing a full spectrum of care as needed.
Wendy Novak Diabetes Institute is part of Norton Healthcare and Norton Children’s.