OBJECTIVE: To compare the risk for pregnancy outcomes by gestational weight gain with the Institute of Medicine criteria and empirically established average ranges of gestational weight gain. METHODS: In a population-based data set comprising 678,560 singleton deliveries in Bavarian obstetric units from 2000 to 2007, we calculated the prevalence of adverse short-term pregnancy outcomes within the gestational weight-gain ranges recommended by the Institute of Medicine. We then compared these for

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... gestational weight gain within data-based interquartile ranges (25th to 75th percentile) and interdecile ranges (10th to 90th percentile) of gestational weight gain by maternal weight category (underweight, normal weight, overweight, and obese). RESULTS: In underweight and normal-weight mothers, adherence to Institute of Medicine criteria was significantly associated with fewer preterm deliveries and small-for-gestational-age births (prevalence [95% confidence interval] for preterm delivery in normal-weight women: 5.33 [5.23-5.43] within Institute of Medicine criteria compared with 5.45 [5.36 -5.54] in interquartile range). Overweight and obese mothers gaining weight within the Institute of Medicine recommendations had less preeclampsia and nonelective caesarean deliveries but had higher risks for gestational diabetes, small-forgestational-age births, preterm delivery, and perinatal mortality compared with gestational weight gain within the respective interquartile ranges and interdecile ranges (prevalence for preterm delivery in overweight women: 8.14% [7.87-8.42] within Institute of Medicine criteria compared with 5.77% [5.60 -5.93] in interquartile range). CONCLUSION: Although underweight and normalweight women should be encouraged to aim for a gestational weight gain according to Institute of Medicine guidelines, different gestational weight gain recommendations in overweight and obese women might lessen some adverse short-term pregnancy outcomes. (Obstet Gynecol 2010;116:1111-8) LEVEL OF EVIDENCE: II G estational weight gain is associated with shortand long-term pregnancy outcomes 1-10 and optimal gestational weight gain appears to vary with pregestational maternal body mass index (BMI). Body mass index is calculated as weight (kg)/[height (m)] 2. Therefore, the US Institute of Medicine established gestational weight gain recommendations for women of different BMI classes in 1990, 11 which were slightly revised in 2009. 12 According to the current guidelines, underweight women (BMI less than 18.5) should gain 12.5-18.0 kg during their pregnancies, normal-weight women (BMI between 18.5 and 25) 11.5-16.0 kg, overweight women (BMI between 25 and 30) 7.0 -11.5 kg, and obese women (BMI 30 or greater) 5.0 -9.0 kg. Although these guidelines are "intended for use among women in the United States," the Institutional of Medicine committee considered them as potentially "applicable to women in