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 IIIobesity to Class Iand 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( RR1.20 [1.13, 1.27]), large‐for‐gestational age was higher ( RR1.37 [1.29, 1.45] and RR1.30 [1.24, 1.36] compared with Class Iand I/ II, respectively), while small‐for‐gestational age was lower ( RR0.89 [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 Ior I/ IIobese women. These findings have important implications for screening and care of morbidly obese pregnant women, to try to decrease adverse outcomes.