Objective: Some methods of family planning may occasionally work after fertilization or implantation. These effects may be important to some women. We explored Spanish women's attitudes towards these potential mechanisms of action of family planning methods.Study design: Cross-sectional study in a Spanish representative sample of 848 potentially fertile women, aged 18-49. Data were collected using a 30-item questionnaire about family planning. Logistic regression was used to identify variables associated with women's attitudes towards postfertilization effects.Results: The majority of women were married, held an academic degree and had at least one child. Forty-five percent of women would not consider using a method that may work after fertilization and 57% would not consider using one that may work after implantation. Fortyeight percent of the sample would stop using a method if they learned that it sometimes works after fertilization, increasing to 63% when referring to a method that sometimes works after implantation. Women who believe that human life begins at fertilization, those who believe it is important to distinguish between spontaneous and induced embryo losses and women who report having a religion were less likely to consider the use of a method with some postfertilization effects. Conclusion:The possibility of postfertilization effects may influence Spanish women's choice of a FP method. Information about mechanisms of action of birth control methods should be disclosed to women so that they can make informed choices.