Objective: Some methods of family planning, such as oral contraceptives, emergency pill or intrauterine device, may occasionally work after fertilization. These effects may be important to some women. We explored Spanish women's attitudes towards contraceptive choices that may have occasional post-fertilization mechanisms of action. 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 post-fertilization effects. Results: The majority of women were married, had completed high school 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. Forty-eight 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 post-fertilization effects.Conclusion:The possibility of post-fertilization effects may influence Spanish women's choice of a family planning method. Information about mechanisms of action of birth control methods should be disclosed to women so that they can make informed choices.