BACKGROUND: The prevalence of overweight and obesity has increased in various countries. Normal weight before pregnancy is important to protect maternal and newborn health. This study aimed to describe the evolution of body mass index (BMI) before pregnancy in France and explore its association with two measures of socioeconomic status (SES), education and household income. METHODS: Data were from four national perinatal surveys in France in 1998, 2003, 2010 and 2016 to describe the time evolution of maternal BMI. We explored the links between BMI and women's characteristics in the most recent period (2010-2016 surveys) since income information was not available before. Risk ratios (RRs) of underweight, overweight and obesity for each measure of SES were computed by using multivariable Poisson regression models. RESULTS: Overweight and obesity prevalence increased between 1998 and 2016, from 6% to 12% for obesity. Both were inversely associated with SES (higher prevalence among least educated and poorest women), with strong variations for each social indicator, even in multivariable analyses including both. Combining education and income revealed a wide gradient; RR for obesity was 6.01 (95% confidence interval 4.89-7.38) with low education and income =4000 euros/month. CONCLUSIONS: Public policies must implement programs to limit the increase in overweight and its unequal distribution in the population, alongside other policies to address the societal determinants of the obesogenic environment. Health professionals need to advise women to improve their eating and physical activity to limit weight gain from childhood to early adulthood.