OBJECTIVEdTo determine associations of gestational diabetes mellitus (GDM) and obesity with adverse pregnancy outcomes in the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Study. RESEARCH DESIGN AND METHODSdParticipants underwent a 75-g oral glucose tolerance test (OGTT) between 24 and 32 weeks. GDM was diagnosed post hoc using Interna- tional Association of Diabetes and Pregnancy Study Groups criteria. Neonatal anthropometrics and cord serum C-peptide were measured. Adverse pregnancy outcomes included birth weight, newborn percent body fat, and cord C-peptide .90th percentiles, primary cesarean delivery, preeclampsia, and shoulder dystocia/birth injury. BMI was determined at the OGTT. Multiple logistic regression was used to examine associations of GDM and obesity with outcomes. RESULTSdMeanmaternalBMIwas 27.7, 13.7%wereobese(BMI $33.0kg/m2), andGDMwas diagnosedin16.1%.Relativetonon-GDMandnonobesewomen,oddsratioforbirthweight .90th percentileforGDMalonewas2.19(1.93–2.47),forobesityalone1.73(1.50–2.00),andforbothGDM and obesity 3.62 (3.04–4.32). Results for primary cesarean delivery and preeclampsia and for cord C-peptideandnewbornpercentbodyfat.90thpercentilesweresimilar.Oddsforbirthweight.90th percentilewereprogressivelygreaterwithbothhigherOGTTglucoseandhighermaternalBMI.There was a 339-g difference in birth weight for babies of obese GDM women, compared with babies of normal/underweightwomen(64.2%ofallwomen)withnormalglucosebasedonacompositeOGTT measure of fasting plasma glucose and 1- and 2-h plasma glucose values (61.8% of all women). CONCLUSIONSdBoth maternal GDM and obesity are independently associated with ad- verse pregnancy outcomes. Their combination has a greater impact than either one alone.