Objective: In longitudinal studies, women gain significant amounts of weight during young adulthood, pointing to pregnancy as an important trigger for weight gain. Studies examining the effect of parity vary in their findings and are complicated by multiple potential confounders. This study examines the association between parity and long-term weight gain in a cohort of young women participating in the Australian Longitudinal Study on Women's Health (ALSWH). Methods: A sample of 8,009 parous and nulliparous women was drawn from this cohort and allocated to one of six parity groups (0-5+). Weight gain and factors associated with BMI ≥ 25 over a 16-year period were identified by using generalized linear equations. Results: Median BMI increased by between 2.95 and 4.9 units over 16 years, with women of parity 5 + showing the biggest gain. Associations between several variables and a BMI ≥ 25 (controlling for multiple demographic and behavioral factors) demonstrated no effect for parity but significant effects for survey year, no paid job, and depression. University education and high levels of physical activity were protective. Conclusions: In this sample, parity was not associated with a BMI ≥ 25 over a 16-year period.