To investigate if the association between maternal pre-pregnancy BMI and offspring’s body composition in late adolescence and young adulthood varies by offspring birth order and sex.

Family cohort study, with data from registers, questionnaires and physical examinations. The main outcome under study was offspring body composition (percentage fat mass (%FM), percentage lean mass (%LM)) measured by dual-energy X-ray absorptiometry.

Uppsala, Sweden.

Two hundred and twenty-six siblings (first-born v. second-born; average age 19 and 21 years) and their mothers.

In multivariable linear regression models, maternal pre-pregnancy BMI was positively associated with daughter’s %FM, with stronger estimates for first-born (β=0·97, 95 % CI 0·14, 1·80) v. second-born daughters (β=0·64, 95 % CI 0·08, 1·20). Mother’s BMI before her first pregnancy was associated with her second-born daughter’s body composition (β=1·05, 95 % CI 0·31, 1·79 (%FM)) Similar results albeit in the opposite direction were observed for %LM. No significant associations were found between pre-pregnancy BMI and %FM (β=0·59, 95 % CI−0·27, 1·44 first-born; β=−0·13, 95 % CI−0·77, 0·52 second-born) or %LM (β=−0·54, 95 % CI−1·37, 0·28 first-born; β=0·11, 95 % CI−0·52, 0·74 second-born) for sons.

A higher pre-pregnancy BMI was associated with higher offspring %FM and lower offspring %LM in late adolescence and young adulthood, with stronger associations for first-born daughters. Preventing obesity at the start of women’s reproductive life might reduce the risk of obesity in her offspring, particularly for daughters.