Given that approximately half of all pregnancies in the United States are unplanned, the authors sought to understand the relation between pregnancy intention and health behaviors. Mothers of live-born infants without major birth defects were interviewed as part of the National Birth Defects Prevention Study. The interview assessed pregnancy intention as well as exposures to vitamins, alcohol, tobacco, illicit drugs, occupational hazards, exogenous heat (e.g., hot tubs and saunas) and caffeine. Crude odds ratios and 95% confidence intervals were calculated and stratified analyses were performed to assess interaction. Multiple logistic regression was used to calculate adjusted odds ratios. Both before and after the diagnosis of pregnancy, women with unintended pregnancies were more likely to use illicit drugs, smoke, be exposed to environmental smoke, and not take folic acid or multivitamins. The degree to which women altered behaviors after they realized they were pregnant was also associated with their pregnancy intention status. For certain behaviors, maternal age or parity altered the association between pregnancy intention and changing behaviors after awareness of pregnancy. Pregnancy intention status is a key determinant of pregnancy-related behavior. To improve reproductive outcomes, preconceptional and prenatal programs should consider a woman's desire for pregnancy.