Based on cognitive dissonance and related theories of commitment, this study tested hypotheses that planning pregnancy and number of months spent trying to conceive would be associated with better prenatal health behaviors and that commitment to pregnancy and motherhood would mediate these associations. Participants (N = 96) were pregnant women at high medical risk for an adverse birth outcome. As predicted, planning pregnancy predicted better prenatal health care practices, and this effect was mediated by commitment level. Among women who planned their pregnancy, longer time to conceive predicted higher commitment but did not influence prenatal health behaviors directly. Women who had given birth previously practiced fewer prenatal health behaviors. Commitment, however, remained the strongest predictor of prenatal health care practices. Results are consistent with theories of commitment and with prominent approaches to the study of health behavior.