Two views on when pregnancy begins are prominent in medical and lay literature. Since 1965 the American College of Obstetricians and Gynecologists (ACOG) has defined pregnancy as starting with implantation of the embryo on the uterus. Many other books, organizations, physicians, and politicians have defined pregnancy as beginning with fertilization of the ovum by the sperm (often called conception). The definition of pregnancy is a critical starting point for debates over which contraceptive technologies should be considered abortifacients. The debate has fresh relevance now that the FDA has approved the “five day pill” (ulipristal acetate), a prescription-only contraceptive that is capable of acting between fertilization and implantation. To examine this question we conducted a national survey of U.S. obstetrician-gynecologists (stratified random sample, n=1800), asking whether they consider pregnancy to begin at conception or at implantation (respondents could also answer “not sure”). Primary predictors were respondents’ religious affiliation, importance of religion, and having a moral objection to abortion. The survey response rate was 66% (1154/1760). 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, doctors who consider religion the most important part of their lives were less likely to believe pregnancy begins at implantation (Odds Ratio 0.5, 95% confidence interval 0.2-0.9), as were doctors who object to abortion (OR 0.4, 95% CI 0.2-0.9). Contrary to ACOG’s nomenclature, a minority of obstetrician-gynecologists define pregnancy as beginning with implantation. This raises important theoretical questions: What is gained/lost by adhering to either definition? Can any group unilaterally decide how a word is used? Would a more nuanced nomenclature add clarity or confusion? Is a consensus possible, given at least 150 years of ambiguity? We propose careful attention to language (on both sides) will help debates become more productive.