In order for conception to occur, a sperm must meet and penetrate an oocyte in a particular portion of the fallopian tube, usually somewhere in the middle third or so. The oocyte reaches this point in the tube between 6 and 24 hours after ovulation. After 24 hours have passed, the egg is no longer capable of being fertilized. It can take sperm a few minutes or several hours to get up into the fallopian tube to the site of the egg. Sperm can also live for 2-4 days inside the reproductive tract of a woman. Thus, it is possible to get pregnant by having sex without contraception around 4 days before ovulation and up to a day or so after ovulation.

If you are trying to get pregnant, doctors usually recommend having sex once per day (more often lowers the amount of sperm in each ejaculation) starting 8-10 days after the start of your last menstruation regardless of how long it lasts. For example, if your period lasts four days, this means that you should start having sex around four days after your period ends. You can continue having sex once per day until day 20 or so, unless you are sure that you have already ovulated. Once you have ovulated, the following two days or so mark the last chance for pregnancy during that cycle.

In general, younger women usually find it easier to become pregnant. As women get older the likelihood of pregnancy decreases until menopause, after which natural pregnancy becomes impossible.