Whether booking body mass index (BMI) in the UK is increasing is unknown but is of clinical interest since overweight or obese pregnant women face far greater risks of pregnancy complications including pre-eclampsia and gestational diabetes. We examined booking BMI in 1990 and 2002/2004, of women with singleton pregnancies. Our analyses indicate an increase of 1 U in mean BMI over this period despite lower parity in recent years. When the model was adjusted for maternal age, parity, smoking status and deprivation category the mean BMI was 1.37 U higher in 2002/2004 than in 1990. More striking was the significant increase in the proportion of women who were obese (BMI > or = 30 kg/m2) at booking--more than twofold higher in unadjusted analysis (18.9% vs 9.4%) rising to greater than threefold higher in multivariate analysis. These findings suggest that obesity-related pregnancy complications are likely to increase with implications for both mother and child.