<qa>  What is the primary cause of unintended pregnancy in the United States?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. 