 Contraceptive failure is the primary cause of unintended pregnancy in the United States. With obesity rates at epidemic proportions, any association between obesity and strategies that prevent undesired pregnancies constitutes a significant public health and economic concern. Evidence from recent epidemiological studies and our preliminary data sub-therapeutic levels of steroid hormones due to drug clearance and half-life suggest that obesity reduces oral contraceptive efficacy. Furthermore, preliminary analysis suggested that a sub-group of obese women, defined by their own birth weight, are at higher risk of contraceptive failure. Further studies are necessary to investigate whether birth weight, a surrogate marker of in utero growth restriction, is a useful diagnostic marker for the identification of women prone to contraceptive failure. Such an understanding is critical to finding a contraceptive strategy with better efficacy for these women. 