Background We aimed to characterise between-women and within-women weight change in adult women, and to assess the association of sociodemographic, reproductive and lifestyle characteristics with the variations.

Methods This study comprised 1040 adult women from a population-based cohort of adults from Porto, Portugal, assembled in 1999–2003 and re-evaluated in 2005–2008 (median follow-up 5 years). A mixed-effects model for longitudinal analysis of body weight was fitted, with random intercepts and random slopes. Multivariate-adjusted coefficients with 95% CIs were computed for the determinants of weight at baseline and weight change between visits.

Results There was an average crude increase of 0.11 kg of measured weight per calendar year (95% CI 0.05 to 0.17). In a multivariate model, despite not explaining differences in weight between women at baseline, years of fertile life were associated with individual weight change: −0.34 kg/year for women with up to 21 years of fertile life, whereas after that, weight increased progressively more with increasing fertile time (21.1–34 years: −0.34 kg+0.42 kg=0.08 kg; 34.1–41 years: −0.34 kg+0.46 kg=0.12 kg) up to 0.44 kg/year above 41 years. Height, age, education, marital status, residential neighbourhood deprivation, number of children delivered and use of oral contraceptives, influenced interindividual weight variation at baseline, but did not influence weight change from baseline to follow-up.

Conclusions Cross-sectional determinants of weight and those of weight change over time are vary. Reproductive factors play important roles in explaining both differences among women as well as in individual weight change over time.