Background: There is an absence of national statistics for maternal obesity in the UK. This study is the first to describe a nationally representative maternal obesity research data set in England. Design: Retrospective epidemiological study of first trimester obesity. Methods: Data from 34 maternity units were analysed, including 619 323 births between 1989 and 2007. Data analysis included trends in first trimester maternal body bass index status over time, and geographical distribution of maternal obesity. Population demographics including maternal age, parity, ethnic group, deprivation and employment were analysed to identify any maternal obesity-associated health inequalities. All demographics were tested for multicollinearity. Logistic regression analyses were adjusted for all demographics as confounders. Results: First trimester maternal obesity is significantly increasing over time, having more than doubled from 7.6% to 15.6% over 19 years (P<0.001), and shows geographic variation in incidence. There are also demographic health inequalities associated with maternal obesity, including increased odds of being obese with increasing age, parity, Black ethnic group and deprivation. There is also an association between morbid obesity and increased levels of unemployment. Conclusions: The increase in maternal obesity has serious implications for the health of mothers, infants and service providers, yielding an additional 47 500 women per year requiring high dependency care in England. The demography of women most at risk of first trimester obesity highlights health inequalities associated with maternal obesity, which urgently needs to be addressed.