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Background Hypertension in pregnancy is an important cause of pregnancy complications for both mother and baby in about 8% of pregnancies. Epidemiological reviews have reported an association between pre-eclampsia and maternal obesity, based on a body mass index (BMI) > 29.9 kg/m2. However, BMI measurements were usually based on self-reporting of maternal weight and height which has been shown to be inaccurate, particularly in pregnancy and in obese women. The purpose of this prospective cross-sectional study was to compare the incidence of pre-eclampsia in the different WHO BMI categories in both primigravidas and multigravidas. Methods Women were recruited at their convenience in the first trimester. Height and weight were measured digitally and BMI calculated. Pre-eclampsia was defined as hypertension (a systolic blood pressure of ≥140 mm Hg and/or a diastolic blood pressure of ≥90 mm Hg) measured on at least two separate occasions at least 6 h apart with proteinuria (≥300 mg over 24 h) after 20 weeks. Results Of the 2230 women, the mean age was 28.2 years and mean parity was 1.0. The mean gestation at recruitment was 11.2 weeks and the mean BMI was 25.6 kg/m2. The incidence of obesity was 17.4%. The incidence of pre-eclampsia analysed by BMI category is show in table 1. In both primigravidas and multigravidas, there was no increase in pre-eclampsia with obesity (p=0.23, p=0.07). Conclusions When maternal BMI is calculated accurately in early pregnancy, contrary to previous reports, there is no association between maternal obesity and pre-eclampsia.