ObjectivesTo compare the management, maternal and perinatal outcomes of women with a body mass index (BMI) ≥60 kg/m²with women with a BMI >50–59.9 kg/m².DesignInternational collaborative cohort study.SettingBinational study in the UK and Australia.ParticipantsUK: all pregnant women, and Australia: women who gave birth (birth weight ≥400 g or gestation ≥20 weeks)MethodsData from the Australasian Maternity Outcomes Surveillance System and UK Obstetric Surveillance System. Management, maternal and infant outcomes were compared between women with a BMI ≥60 kg/m²and women with a BMI >50–59.9 kg/m², using unconditional logistic regression.ResultsThe sociodemographic characteristics and previous medical histories were similar between the 111 women with a BMI ≥60 kg/m²and the 821 women with a BMI >50–59.9 kg/m². Women with a BMI ≥60 kg/m²had higher odds of thromboprophylaxis usage in both the antenatal (24% vs. 12%; OR 2.25, 95% CI 1.39 to 3.64) and postpartum periods (78% vs. 66%; OR 1.68, 95% CI 1.04 to 2.70). Women with BMI ≥60 kg/m²had nearly double the odds of pre-eclampsia/eclampsia (adjusted OR 1.83 (95% CI 1.01 to 3.30)). No other maternal or perinatal outcomes were statistically significantly different. Severe adverse outcomes such as perinatal death were uncommon in both groups thus limiting the power of these comparisons. The rate of perinatal deaths was 18 per 1000 births for those with BMI ≥60 kg/m²; 12 per 1000 births for those with BMI >50–59.9 kg/m²; those with BMI ≥60 kg/m²had a non-significant increased odds of perinatal death (unadjusted OR 1.46, 95% CI 0.31 to 6.74).ConclusionsWomen are managed differently on the basis of BMI even at this extreme as shown by thromboprophylaxis. The pre-eclampsia result suggests that future research should examine whether weight reduction of any amount prior to pregnancy could reduce poor outcomes even if women remain extremely obese.