Why when the male cums in you you don't get pregnant?

Sperm and egg needed: Most women ovulate (release an egg) approx. 12-16 days after the 1st day of menses. If u r not ovulating around the same time u have sex, it is not likely that u will become pregnant. Women who r not on contraception & who have regular intercourse have about a 1 in 5 (20%) chance of getting pregnant in any given month. In other words, even if your timing is perfect, u r not guaranteed 2 get pregn.