OBJECTIVE We investigated how average weekly gestational weight gain rates during three periods of pregnancy were related to the offspring's body mass index (BMI) at 7 years of age. METHODS We used data from the Danish National Birth Cohort. Information from pregnancy interviews conducted between week 12-20 (Interview 1) and week 25-32 (Interview 2) was used to calculate maternal weight gain in the following periods: up until Interview 1; between Interview 1 and 2; from Interview 2 until delivery. Life Course Path Analyses were used to obtain effect estimates of the rates of gain on the offspring's BMI. RESULTS Average weekly gestational weight gain rates in first and second period of pregnancy were significantly associated with the child's BMI (ß-coefficient (95% CI): 0.049 z-score/z-score (0.030, 0.067), and 0.059 z-score/z-score (0.041, 0.077), respectively). No significant association between weight gain in the third period and the child's BMI was found (ß-coefficient (95% CI): 0.016 z-score/z-score (-0.002, 0.034)). The total effect estimate from the third period was furthermore significantly lower than the effect estimates from the first and second period (p < 0.03 and p < 0.002, respectively). CONCLUSIONS Gestational weight gain in first and second, but not in the last period of pregnancy were positively associated with the offspring's BMI at 7 years of age, even though the estimates were small. Fetal environmental factors may influence the unborn child differently depending on timing of exposure. Therefore, it may be important to consider the different fetal developmental stages when linking early life experiences with later health.