Half of the placental genes to which a woman is exposed during pregnancy come from her mating partner. Placental hormones, especially human chorionic gonadotropin and human placental lactogen, are considered to mediate the protective effects of full-term pregnancy and lactation on breast cancer risk. In this paper, variants in a woman’s placental human chorionic gonadotropin or human placental lactogen genes, which are easily measurable through her offspring’s genotypes, are associated with her breast cancer risk. If this hypothesis is true it would indicate that genotype of a woman’s mating partner can affect her breast cancer risk and that offspring’s genotype may be useful in predicting such risk. Because the placenta produces a wide range of hormones and enzymes (in addition to human chorionic gonadotropin and human placental lactogen), results supporting this hypothesis could open new dimensions to genetic research for diseases beyond breast cancer (including gynecologic tumors and reproductive and pregnancy-related disorders).