text: why don't women get pregnant when they take a birth control pill?   birth control pills usually contain the hormones progesterone and/or estrogen. a progesterone-only pill (statistically) has less side effects than the progesterone & estrogen pill. the hormones prevent the uterous from ovulating. therefore, the egg will never be released into the fallopian tube, where it can encounter the sperm. it is important to take the pill several days before vaginal intercoarse because it takes awhile for the hormones to take effect.  otherwise, happy hunting!