The aim of our study was to compare the impact of maternal obesity on the incidence of medical complications of pregnancy in France and Canada. We performed a prospective comparative cohort study using French data, retrieved from a prospective cohort of singleton deliveries, and Canadian data retrieved from QUARISMA, a cluster-randomized controlled trial conducted in Quebec, both between 2009 and 2011. Outcomes studied included, hypertensive disorders of pregnancy (HDP), venous thromboembolism, stillbirth, caesarean delivery and macrosomia. The impact of obesity across both cohorts was studied using univariate and multivariate logistic regression analyses, adjusting for relevant confounders. The French and Canadian databases included 26,973 and 22,046 deliveries respectively, with obesity rates of 9.1% and 16% respectively (p < 0.001). In both cohorts, obesity was significantly associated with an increased rate of HDP, cesarean delivery, and macrosomia. However, in both cohorts the relationship between increasing body mass index and the incidence of medical complication of pregnancy was the same, regardless the outcome studied. In conclusion, obesity is a risk factor for adverse maternal and fetal outcomes in both cohorts. Similar trends of increased risk were noted in both cohorts even though obesity is more prevalent in Canada. The prevalence of obesity has dramatically increased over the past 30 years, especially in North America. This condition is now described as an " epidemic " , and has consistently been associated to adverse health outcomes such as cardiovascular disorders, metabolic disorders, the occurrence of certain cancers, as well as psychoso-cial problems 1. The impact of obesity on life expectancy is considerable: in nonsmoking Americans, obesity at 40 years of age is associated with a life expectancy reduction of 7.1 years for women and 5.8 years for men 2. In Canada, the rate of obesity among adult women of reproductive age has increased from 16% in 1997, to 23.9% in 2009 (+50%) whereas the increase has been 82% in the same period in France, from 8.3% to 15.1% 3–5. A similar trend has been observed in pregnant women: in 2009, 16% of women starting a pregnancy were obese in Canada, in comparison to 9.9% in France 6, 7. This expansion is both linked to a general trend of increasing obesity but also to an increase in the age of childbearing in recent years; indeed, obesity also increases with advancing maternal age 4, 7. Consequently, obesity appears to be more longstanding and more prevalent in Canada than in France. Definitions of obesity have varied widely over time. The most frequent definition utilized is based on the Body Mass Index (BMI), a simple measure of weight-for-height defined as the weight in kilograms divided by the square of the height in meters (kg/m 2). The ranges are independent of age, parity, smoking history, race and ethnic background 8–10. Given the growing prevalence of increased BMI and the difference in obesity rates between Canada and France, we aimed to study the relationship between obesity and the incidence of medical complications of pregnancy in those two countries. We therefore compared pregnancy outcomes according to BMI classes between the two countries, through a secondary analysis of data collected in two well-defined contemporaneous cohorts.