Morbidly obese (Class III, body mass index [BMI] ≥ 40 kg m−2) women constitute 8% of reproductive-aged women and are an increasing proportion; however, their pregnancy risks have not yet been well understood. Hence, we performed meta-analyses following the MOOSE (Meta-Analysis of Observational Studies in Epidemiology) guideline, searching Medline and Embase from their inceptions. To examine graded relationships, we compared Class III obesity to Class I and I/II, and separately to normal weight. We found important effects on all three primary outcomes in morbidly obese women: preterm birth <37 weeks was 31% higher compared with Class I (relative risk [RR] 1.31 [1.19, 1.43]) and 20% higher than Class I/II (RR 1.20 [1.13, 1.27]), large-for-gestational age was higher (RR 1.37 [1.29, 1.45] and RR 1.30 [1.24, 1.36] compared with Class I and I/II, respectively), while small-for-gestational age was lower (RR 0.89 [0.84, 0.93] compared with Class I, with nearly identical reductions for Class I/II). Morbidly obese women have higher risks of preterm birth, large-for-gestational age and numerous other adverse maternal and infant health outcomes, relative to not only normal weight but also Class I or I/II obese women. These findings have important implications for screening and care of morbidly obese pregnant women, to try to decrease adverse outcomes.