What you can expect

Using the traditional calendar rhythm method involves these steps:

• Record the length of six to 12 of your menstrual cycles. Using a calendar, write down the number of days in each menstrual cycle — counting from the first day of your period to the first day of your next period.
• Determine the length of your shortest menstrual cycle. Subtract 18 from the total number of days in your shortest cycle. This number represents the first fertile day of your cycle. For example, if your shortest cycle is 26 days long, subtract 18 from 26 — which equals 8. In this example, the first day of your cycle is the first day of menstrual bleeding and the eighth day of your cycle is the first fertile day.
• Determine the length of your longest menstrual cycle. Subtract 11 from the total number of days in your longest cycle. This number represents the last fertile day of your cycle. For example, if your longest cycle is 32 days long, subtract 11 from 32 — which equals 21. In this example, the first day of your cycle is the first day of menstrual bleeding and the 21st day of your cycle is the last fertile day.
• Plan sex carefully during fertile days. If you're hoping to avoid pregnancy, unprotected sex is off-limits during your fertile days — every month. On the other hand, if you're hoping to get pregnant, have sex regularly during your fertile days.
• Update your calculations every month. Continue recording the length of your menstrual cycles to make sure you're properly determining your fertile days.

Keep in mind that many factors, including medications, stress and illness, can affect the exact timing of ovulation. Using the rhythm method to predict ovulation can be inaccurate, especially if your cycle is irregular.

Standard Days method

A more modern approach to the calendar rhythm method is the Standard Days method. This method works best if your menstrual cycles are usually between 26 and 32 days long.

To use the Standard Days method:

• Count the days in your menstrual cycle, starting with the first day of your period as day 1. Continue counting each day of your cycle until your next cycle starts.
• On days 1-7, you're not considered to be fertile and can have unprotected sex, though you may have menstrual bleeding on those days.
• On days 8-19, you're considered to be fertile. Avoid unprotected sex or abstain from sex to avoid pregnancy. Or, if you're trying to get pregnant, these are the days to have unprotected sex.
• On day 20 through the end of your cycle, you're no longer fertile and can have unprotected sex.
• When you get your next period, start the counting over at day 1.

To help you keep track of your cycle days, you can use a tool such as CycleBeads — a color-coded string of beads that allows you to identify fertile and nonfertile days. Or you can use the CycleBeads app to keep track of cycle days. From the app, you can also learn whether the Standard Days method is appropriate for you, based on the length of your menstrual cycles.