A woman SHEDS her uterine lining every month. AKA a period. (this is the "inside of the womb" in which an embryo would embed itself.) There is no way little dead embryos would be able to stick around, even if the pill did stop implantation.

In fact, if an embryo dies after implantation, that is a miscarriage, in which the woman's uterine lining sheds, taking the dead embryo with it. Bio 101.