Background: Women often blame weight gain in early adulthood on having a baby. Purpose: The aim was to estimate the weight gain attributable to having a baby, after disentangling the effects of other factors that influence weight change at this life stage. Methods: A longitudinal study of a randomly selected cohort of 6458 Australian women, aged 18-23 years in 1996, was conducted. Self-report mailed surveys were completed in 1996, 2000, 2003, and 2006, and data were analyzed in 2008. Results: On average, women gained weight at the rate of 0.93% per year (95% CI=0.89, 0.98) or 605 g/year (95% CI=580, 635) for a 65-kg woman. Over the 10-year study period, partnered women with one baby gained almost 4 kg more, and those with a partner but no baby gained 1.8 kg more, than unpartnered childless women (after adjustment for other significant factors: initial BMI and age; physical activity, sitting time, energy intake (2003); education level, hours in paid work, and smoking). Conclusions: Having a baby has a marked effect on 10-year weight gain, but there is also an effect attributable to getting married or living with a partner. Social and lifestyle as well as energy balance variables should be considered when developing strategies to prevent weight gain in young adult women.