If you have an absolutely regular twenty-eight-day cycle, you will ovulate around day 14 (counting the first day of your menstrual period as day 1). If you have a longer cycle, you will probably ovulate a little later. If you have a shorter cycle, you will probably ovulate earlier.

However, the time between ovulation and the beginning of the next period remains about fourteen days no matter how long or short your cycle; it is the beginning of the cycle whose length varies. So if you have a thirty-five-day cycle, you will probably ovulate on day 21, whereas if you have a twenty-five-day cycle, you are likely to ovulate on day 11. This means that if you know when you ovulate, it is easy to predict when you will get your period. But it does not mean that if you know when your last period began, you know when you are next going to ovulate.

How does the calendar method work?

People who use the calendar or rhythm method in its plainest version use a calendar to predict future menstrual cycles, assuming they will be like past ones. These observations can help you guess a couple of days in advance when you probably will ovulate, though no one is absolutely regular every month every year. Use this method in conjunction with the basal body temperature method and/or the cervical mucus method, and if any sign suggests that you might be ovulating, abstain from vaginal intercourse (or use a backup method of contraception).

Kimberley usually has a menstrual cycle that varies between twenty-five and thirty days. If her shortest period is twenty-five days, then she probably ovulates no earlier than day n; if her longest cycle is thirty days, then she can ovulate as late as day 16. Her fertile period is likely to be between days 11 and 16.

However, because sperm can still fertilize an egg for two or three days after they are ejaculated, to be on the safe side, she should add a couple of days to day 16. And maybe she is ovulating just a littler earlier than she thinks, so she should add a day or so to the beginning of the fertile period, pushing the starting date back to day 9. So between days 9 and 18, Kimberley and her partner should abstain from unprotected intercourse.

How do you find your safe days?

On a regular calendar, circle day 1 of your period, the day bleeding begins. Continue to circle day 1 every month for at least eight months, preferably a year. To determine your safe days, find the shortest cycle on your calendar. Subtract 18 from the total number of days: If your shortest cycle was 26 days, then 18 from 26 leaves 8. Starting with day 1 of your period, count forward 8 days. Mark that date on your calendar with an X or a special color. It is the first day you are likely to be fertile. You should abstain from intercourse beginning on that date (or use backup contraception). The longer the time of abstinence, the more reliable the method. Some people suggest subtracting 21 instead of 18; although you will be safer, you will be able to have intercourse on fewer days.

To find your last fertile day, subtract 11 days from your longest cycle. Say it was 30 days, so that leaves 19. Mark that date on your calendar as well, with another X or another colored mark. It is the last day you must abstain or use backup contraception.

Your safe days are between the Xs (or the colored marks). The other days are unsafe. If any other sign — a change in cervical mucus, or a drop in the basal body temperature, or spotting, or a twinge of ovarian pain — suggests that you may be ovulating, then you must abstain or use backup contraception until you have reached the next safe day.

Why does the rhythm method fail?

If your cycles are absolutely regular and you conscientiously avoid intercourse on your fertile days, the method works reasonably well. It fails when you ovulate much earlier (or later) than you think you will, or when sperm remain potent much longer than you anticipate.


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