The rhythm method of contraception
Q: At what point in a woman’s cycle can she become pregnant? I heard that pregnancy can only occur three days before and three days after her period. Is this true, and if not when can one get pregnant?
A: Basically, you can become pregnant only around the time of ovulation. Most women ovulate about 14 days before they get their next period. So a woman who has a 28-day cycle will ovulate around day 14, counting the first day of the period as day 1 of the cycle. If she has a 35-day cycle, she will probably be ovulating around day 21 of the cycle.
The problem is that very few women have 100% predictable cycles. Even if you normally have 30-day cycles, for example, it is quite possible that one cycle could be 24 days. So let’s say you thought you couldn’t get pregnant until day 16 of the cycle, and were having unprotected intercourse until day 14. But this cycle you happened to ovulate on day 10. Well, guess what – you might well have conceived when you thought you were “safe.”
You also need to remember that sperm can hang around for a while, and are capable of fertilizing an egg for a day or two after they are deposited in the vagina. And the egg itself can be fertilized for a day or so after it is “hatched.” So this translates to a relatively long window of possible fertile times. These calculations are also the cornerstone of the rhythm method of birth control. If you have reasonably regular cycles, 28 days for example, I would recommend that you not have unprotected intercourse from day 10 to day 18, if you do not want to conceive. If you are trying to conceive, focus your intercourse on those days (once a day or every other day).
Remember that this method of contraception is only about 85% reliable. A couple that does not want to conceive must use a real method of contraception as well. If you would just prefer to delay pregnancy right now, but it would be okay if you did conceive, then the rhythm method is very reasonable.