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Am Fam Physician. 2023;108(3):249-258

This clinical content conforms to AAFP criteria for CME.

Author disclosure: No relevant financial relationships.

Gestational diabetes mellitus (GDM) is a common condition of pregnancy with increasing prevalence in the United States. GDM increases risks of complications, including operative delivery, hypertensive disorders, shoulder dystocia, fetal macrosomia, large-for-gestational-age infants, neonatal hypoglycemia, and neonatal respiratory distress. In patients who are overweight or obese, prepregnancy weight loss and lifestyle modifications during pregnancy may prevent GDM. First-trimester screening can identify preexisting diabetes and early-onset GDM for prompt implementation of glucose control measures. Treatment of GDM has been shown to reduce the risk of complications and should start with lifestyle modifications. For patients who are unable to maintain euglycemia with lifestyle modifications alone, insulin is the recommended first-line medication. For patients with poor glucose control or who require medications, fetal surveillance is suggested starting at 32 weeks of gestation. For all patients with GDM, physicians should assess for fetal macrosomia (estimated fetal weight more than 4,000 g) and discuss the risks and benefits of prelabor cesarean delivery if the estimated fetal weight is more than 4,500 g. Delivery during the 39th week of gestation may provide the best balance of maternal and fetal outcomes. The recommended delivery range for patients controlling their glucose levels with lifestyle modifications alone is 39/0 to 40/6 weeks of gestation, and the ideal range for those controlling glucose levels with medications is 39/0 to 39/6 weeks of gestation. Practice patterns vary, but evidence suggests that glucose management during labor can safely include decreased glucose testing and sliding-scale dosing of insulin as an alternative to a continuous intravenous drip. Insulin resistance typically resolves after delivery; however, patients with GDM have an increased risk of developing overt diabetes. Continued lifestyle modifications, breastfeeding, and use of metformin can reduce this risk.

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

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.
A meta-analysis of four randomized controlled trials and 13 observational studies found no difference in large-for-gestational-age infants between one-step and two-step GDM screening methods, but patients screened with the one-step method experienced higher rates of hypoglycemia and infants admitted to the neonatal intensive care unit. The one-step method results in twice as many GDM diagnoses without improving neonatal or maternal outcomes.
A meta-analysis of 17 studies found that, compared with insulin, metformin leads to lower rates of macrosomia, neonatal hypoglycemia, neonatal intensive care unit admission, cesarean delivery, and pregnancy-induced hypertension in patients with GDM.
Clinical recommendation Evidence rating Comments
Pregnant patients should exercise and receive lifestyle advice to prevent GDM.7 C Low-quality, disease-oriented evidence
Screen pregnant patients without known diabetes for GDM at 24 to 28 weeks of gestation.1,2,4 B Practice guidelines based on systematic reviews; U.S. Preventive Services Task Force B recommendation
Consider screening for preexisting and early-onset GDM at initiation of pregnancy care in patients who are overweight or obese and have one additional diabetes risk factor.1,4 C Practice guidelines based primarily on expert opinion
Initial treatment of GDM should include glucose monitoring and targeted education on nutrition and exercise.4,25,31,32,34,35 C Practice guidelines based on meta-analysis of RCTs
Although insulin is the preferred medication for treating GDM, metformin may be used instead after discussion of potential risks and benefits; glyburide can be considered as a third-line option.4,25,27,4144 B Meta-analyses demonstrating improved perinatal outcomes with metformin; there is inconsistency among guidelines due to concerns of potential long-term risks
Consider induction at or after 39 weeks of gestation for patients with GDM. Induction is recommended before 41 weeks in those with diet-controlled GDM and before 40 weeks in those with medication-controlled GDM.4,5962 C Consensus guidelines; RCTs demonstrating improved outcomes with induction around 39 weeks of gestation
Consider recommending intensive lifestyle changes, administration of metformin, or both for patients with impaired glucose tolerance after a pregnancy complicated by GDM to reduce the risk of progression to type 2 diabetes.71 C RCT demonstrating reduction of progression to diabetes over 10 years with both metformin and lifestyle interventions


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 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

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