For example, if your shortest cycle is 26 days long, subtract 18 from 26 - which equals 8.

This number represents the first fertile day of your cycle. Subtract 18 from the total number of days in your shortest cycle.

Determine the length of your shortest menstrual cycle.

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.

Record the length of six to 12 of your menstrual cycles.

Using the traditional calendar rhythm method involves these steps:

You've recently stopped taking birth control pills or other hormone contraceptives.

However, if you want to use the rhythm method for birth control, talk to your health care provider first if: Tracking your menstrual history doesn't require special preparation. The rhythm method doesn't protect you from sexually transmitted infections. In general, as many as 24 out of 100 women who use natural family planning for birth control become pregnant the first year.

How well the rhythm method works varies between couples. However, it's considered one of the least effective forms of birth control. Using the rhythm method as a form of birth control doesn't pose any direct risks. The rhythm method is an inexpensive and safe way to help you chart your fertility - the time of month when you're most likely to be able to get pregnant. Some women choose to use the rhythm method if a complex medical history limits traditional birth control options, or for religious reasons. The rhythm method can be used as a way to promote fertility or as a method of contraception, by helping you determine the best days to have or avoid unprotected sex.

Show more products from Mayo Clinic Why it's done Book: Mayo Clinic Guide to Fertility and Conception.Book: Mayo Clinic Guide to a Healthy Pregnancy.

What does t minus 3 days mean