I'd like to add my two cents here. Many BC pills have a secondary backup that makes the uterus inhospitable to the developing baby, and your body expels it during your period.The PDR uses language such as "a progestational effect on the endometrium, interfering with implantation." (Ortho-Novum). It is possible to get pregnant while on the pill (I did, twice) and the pill cause miscarriage. Sometimes, this can cause excessive bleeding requiring d&c.