OBJECTIVE: To examine the association between maternal BMI>50kg/m2 during pregnancy and maternal and perinatal outcomes. MATERIALS AND METHODS: An international cohort study was conducted using data from separate national studies in the UK and Australia. Outcomes of pregnant women with BMI>50 were compared to those of pregnant women with BMI<50. Multivariable logistic regression estimated the association between BMI>50 and perinatal and maternal outcomes. RESULTS: 932 pregnant women with BMI>50 were compared with 1232 pregnant women with BMI<50. Pregnant women with BMI>50 were slightly older, more likely to be multiparous, and have pre-existing comorbidities. There were no maternal deaths, however, extremely obese women had a nine-fold increase in the odds of thrombotic events compared to those with a BMI<50 (uOR: 9.39 (95%CI:1.15-76.43)). After adjustment, a BMI>50 during pregnancy had significantly raised odds of preeclampsia/eclampsia (aOR:4.88(95%CI: 3.11-7.65)), caesarean delivery (aOR: 2.77 (95%CI: 2.31-3.32)), induction of labour (aOR: 2.45(95% CI:2.00-2.99)) post caesarean wound infection (aOR:7.25(95%CI: 3.28-16.07)), macrosomia (aOR: 8.05(95%CI: 4.70-13.78)) compared a BMI<50. Twelve of the infants born to women in the extremely obese cohort died in the early neonatal period or were stillborn. CONCLUSIONS: Pregnant women with BMI>50 have a high risk of inferior maternal and perinatal outcomes.