OBJECTIVE: The purpose of this study was to assess obstetrician-gynecologists' regarding their beliefs about when pregnancy begins and to measure characteristics that are associated with believing that pregnancy begins at implantation rather than at conception. STUDY DESIGN: We mailed a questionnaire to a stratified, random sample of 1800 practicing obstetrician-gynecologists in the United States. The outcome of interest was obstetrician-gynecologists' views of when pregnancy begins. Response options were (1) at conception, (2) at implantation of the embryo, and (3) not sure. Primary predictors were religious affiliation, the importance of religion, and a moral objection to abortion. RESULTS: The response rate was 66% (1154/1760 physicians). One-half of US obstetrician-gynecologists (57%) believe pregnancy begins at conception. Fewer (28%) believe it begins at implantation, and 16% are not sure. In multivariable analysis, the consideration that religion is the most important thing in one's life (odds ratio, 0.5; 95% confidence interval, 0.2-0.9) and an objection to abortion (odds ratio, 0.4; 95% confidence interval, 0.2-0.9) were associated independently and inversely with believing that pregnancy begins at implantation. CONCLUSION: Obstetrician-gynecologists' beliefs about when pregnancy begins appear to be shaped significantly by whether they object to abortion and by the importance of religion in their lives.