Pregnancy is thought to be a major contributor to the excess prevalence of obesity in women compared to men. Pregnancy-related increases in weight are purported to increase the risk that women will develop chronic diseases associated with high body weight. The assertion that pregnancy is associated with permanent weight gain and overweight was examined among 41184 post-menopausal women participating in a population-based study. Women reported lifetime parity, weight at ages 18, 30, 40 and 50 years, and current height. Body weight and body mass index (BMI) increased with age. On average, women gained 11.05 kg, or 0.35 kg per year between the ages of 18 and 50 years. Parity was associated with an increase in body weight from age 18 to 50 years of 0.55 kg per live birth, or 0.09 kg per live birth per year. At each age, women with lifetime parity of one or two live births had lower mean body weight and BMI, and a lower proportion overweight (BMI greater than 27 kg/m2), than either nulliparous women or those with three or more lifetime births. These results indicate a strong association between ageing and weight gain and a weak association between parity and both weight gain and overweight in women.