Unprecedently high rates of gestational diabetes in women with body mass index ≥40kg/m 2 at booking: a retrospective UK cohort study Dear Editor, The rising prevalence of maternal obesity, defined as a body mass index (BMI) ≥30kg/m 2 , is a major public health concern. The Royal College of Obstetricians and Gynaecologists estimates that 21.3% of the UK antenatal population are obese, and the recent Confidential Enquiry into the 209 maternal deaths that occurred in the UK from 2015-2017 shows that over a third (34.4%) of the women who died were obese[1].