Early life factors such as birth weight have been associated with the risk of disease in adulthood, including osteoporosis. In the United States, an estimated eight million women have osteoporosis, a disease characterized by low bone mass and associated with increased risk of fracture. Peak bone mass, achieved during early adulthood, is a key determinant of risk of subsequent osteoporosis. Prior studies have suggested that an individual's birth weight is positively associated with bone mineral content (BMC) and bone mineral density (BMD) but results have differed depending on site of bone measurement and other factors considered. We assessed the relationship between birth weight and BMC and BMD using data from the University of Massachusetts Vitamin D Status Study, a cross-sectional study of 186 US women aged 18 to 30 years. Birth weight was assessed via self report and BMC and BMD were measured by dual energy x-ray absorptiometry (DXA). Multivariable linear regression and multivariable logistic regression were used to model the association between birth weight and BMC and BMD, adjusting for established risk factors for low bone density. After controlling for important factors, birth weight was positively associated with BMC and BMD, in large part due to the strong relationship between birth weight and body size. A better understanding of the physiology of the association between birth weight and adult body size and peak bone mass is needed to determine if birth weight is independently associated with peak bone mass.