Gestational diabetes mellitus (GDM) is a form of glucose intolerance first recognised during pregnancy. It occurs when the body cannot produce enough insulin to meet the increased demands of pregnancy, resulting in elevated blood glucose levels. GDM affects between 5% and 15% of pregnancies depending on the diagnostic criteria used and the population studied — and rates are increasing globally alongside rising rates of obesity and type 2 diabetes in women of reproductive age.1
When and how screening happens
All major guidelines recommend universal screening for GDM between 24 and 28 weeks of gestation.2 3 Women with risk factors (obesity, family history of diabetes, previous GDM, age over 25–30, PCOS, or high-risk ethnic background) may be screened earlier, at the first prenatal visit, to detect pre-existing type 2 diabetes that was undiagnosed before pregnancy.
There are two main screening approaches, and the difference matters:
The two-step approach (recommended by ACOG): Step one is a 50-gram non-fasting glucose challenge test. If the one-hour blood glucose is 130–140 mg/dL or above (threshold varies by practice), step two follows: a fasting 100-gram, three-hour oral glucose tolerance test (OGTT). GDM is diagnosed if two or more of the four glucose values exceed the thresholds.2
The one-step approach (recommended by ADA and IADPSG): A fasting 75-gram, two-hour OGTT. GDM is diagnosed if any one of the three glucose values exceeds the threshold: fasting 92 mg/dL or above, one-hour 180 mg/dL or above, or two-hour 153 mg/dL or above.3
The one-step approach diagnoses more cases (approximately 11.5% vs 4.9%), but USPSTF data shows no clear difference in pregnancy, foetal, or neonatal outcomes between the two approaches — raising the question of whether the lower thresholds lead to overdiagnosis and overtreatment in some cases.4
What GDM means for your pregnancy
Untreated GDM increases the risk of macrosomia (large-for-gestational-age baby), shoulder dystocia during delivery, neonatal hypoglycaemia, neonatal jaundice, pre-eclampsia, and caesarean delivery. The risks increase continuously with maternal glucose levels — there is no sharp threshold below which risk disappears.3
Importantly, treatment works. Evidence shows that treating GDM reduces macrosomia and shoulder dystocia, with a number needed to treat of 34. The USPSTF found moderate net benefit for reducing pre-eclampsia, macrosomia, and shoulder dystocia when screening occurs after 24 weeks.4
Management: what actually happens
Blood glucose monitoring. Self-monitoring of blood glucose is the foundation of GDM management. ACOG recommends target values of: fasting or pre-meal below 95 mg/dL, one-hour postprandial below 140 mg/dL, and two-hour postprandial below 120 mg/dL.2
Dietary modification. The first-line treatment is carbohydrate-controlled eating — not calorie restriction. A registered dietitian or nutritionist should provide individualised guidance on distributing carbohydrate intake across meals and snacks to minimise glucose spikes while maintaining adequate nutrition for pregnancy.
Physical activity. The ADA 2026 Standards of Care recommend 30 minutes of moderate-intensity aerobic exercise at least five days per week (minimum 150 minutes weekly). Research shows that walking for 10–15 minutes after each meal can significantly improve postprandial glucose levels.3
Medication. If blood glucose targets cannot be met through diet and exercise within one to two weeks, pharmacological treatment is added. Insulin is the preferred first-line medication because it does not cross the placenta. Metformin and glyburide are used in some settings, though ACOG and SMFM have both recently released guidance on the relative evidence for insulin versus metformin.2
After delivery
GDM typically resolves after delivery, but it signals significantly elevated long-term risk. The ADA 2026 Standards recommend screening for persistent diabetes or pre-diabetes at 4–12 weeks postpartum using a 75-gram OGTT, followed by lifelong screening every one to three years.3 Women with a history of GDM have a 50% or greater lifetime risk of developing type 2 diabetes. This is not a postscript — it is a critical piece of long-term health planning that is frequently overlooked once the baby arrives.
