Gestational diabetes mellitus (GDM) is glucose intolerance that is first recognized during pregnancy and suggests underlying beta cell dysfunction.¹ GDM affects up to 1 in 11 pregnancies in the United States.² As rates of inactivity, overweight/obesity, and advanced maternal age have increased, so has the incidence of GDM, which nearly doubled from 2006 to 2017.³ GDM increases the risk of numerous obstetric complications, including operative delivery, hypertensive disorders, shoulder dystocia, fetal macrosomia (estimated fetal weight more than 4,000 g), large-for-gestational-age (LGA) infants (those above the 90th percentile in weight for gestational age), neonatal hypoglycemia, and respiratory distress. Adequate glucose control reduces many of these risks.³ Patients requiring only lifestyle changes to achieve euglycemia are designated as having A1GDM, whereas those requiring medications are designated as having A2GDM.⁴

Prevention

Studies examining weight changes among pregnancies suggest that even a preconception decrease in body mass index (BMI) of only 1 kg per m² reduces the risk of developing GDM.⁵ Although prepregnancy weight loss appears to be effective, weight loss during pregnancy is not recommended due to the risk of small-for-gestational-age infants.⁶ A Cochrane review of interventions during pregnancy concluded that receiving lifestyle advice and exercising can reduce the risk of GDM, but either intervention alone is not effective.⁷ Additionally, this review noted only low-quality evidence supporting the use of vitamin D and myo-inositol for GDM prevention.⁷ High‐quality studies are needed to investigate the effect of myo‐inositol, vitamin D, probiotics, metformin, and dietary and exercise interventions on the development of GDM.

Screening

Screening for GDM in pregnant patients without known diabetes is recommended at 24 to 28 weeks of gestation.^1,2,4 Retrospective observational studies of second-trimester screening have shown that it reduces the risk of cesarean delivery, birth injury, stillbirth, and admission to a neonatal intensive care unit.² Repeat screening during pregnancy is not generally recommended but may be warranted if a high-risk condition such as polyhydramnios develops.⁸ Recommended screening options include the one- and two-step methods, which are similarly effective.^4,9

The one-step method consists of a 75-g oral glucose tolerance test (OGTT) with glucose measurements after one and two hours. Diagnostic criteria for the 75-g OGTT are set by the International Association of Diabetes and Pregnancy Study Group; a positive result requires only one abnormal value.¹⁰

The two-step method consists of a 50-g OGTT with a glucose measurement after one hour. If the result is elevated, a diagnostic fasting 100-g OGTT is conducted, with glucose measurements each hour for three additional hours.^4,10 For the initial 50-g OGTT, a positive result varies between 130 and 140 mg per dL (7.21 to 7.77 mmol per L), with institution-specific cutoffs.⁴ A threshold of 135 mg per dL (7.49 mmol per L) has 93% sensitivity and 79% specificity; this is similar to the specificity of the 140 mg per dL threshold (82%).² For patients with one-hour glucose values of greater than 199 mg per dL (11.04 mmol per L), forgoing the confirmatory 100-g OGTT and presumptively treating for GDM can be considered.¹¹ The Carpenter-Coustan criteria or the National Diabetes Data Group threshold can be used to determine abnormal 100-g OGTT values. GDM is diagnosed if the measurement has two values at or above the chosen threshold, but even an elevation of one value may indicate increased risk of associated complications.⁴