Context: Prevalence of gestational diabetes mellitus (GDM) and obesity continue to increase.Objective: This study aimed to ascertain whether diet and exercise is a successful intervention for women with GDM and whether a subset of these women have comparable outcomes to those with normal glucose tolerance (NGT).Design, Setting, and Participants: This was a retrospective cohort study of five antenatal centers along the Irish Atlantic seaboard of 567 women diagnosed with GDM and 2499 women with NGT during pregnancy.Intervention: Diet and exercise therapy on diagnosis of GDM were prescribed and multiple maternal and neonatal outcomes were examined.Results: Infants of women with GDM were more likely to be hypoglycemic (adjusted odds ratio [aOR], 7.25; 95% confidence interval [CI], 2.94-17.9) at birth. They were more likely to be admitted to the neonatal intensive care unit (aOR, 2.16; 95% CI, 1.60-2.91). Macrosomia and large-forgestational-age rates were lower in the GDM group (aOR, 0.48; 95% CI, 0.37-0.64 and aOR, 0.61; 95% CI, 0.46-0.82, respectively). There was no increase in small for gestational age among offspring of women with GDM (aOR, 0.81; 95% CI, 0.49-1.34). Women with diet-treated GDM and body mass index (BMI) <25 kg/m(2) had similar outcomes to those with NGT of the same BMI group. Obesity increased risk for poor pregnancy outcomes regardless of diabetes status.Conclusion: Medical nutritional therapy and exercise for women with GDM may be successful in lowering rates of large for gestational age and macrosomia without increasing small-for-gestational- age rates. Women with GDM and a BMI less than 25kg/m(2) had outcomes similar to those with NGT suggesting that these women could potentially be treated in a less resource intensive setting.