What is the rhythm method?

Bottom lineThe rhythm method is the 1930s calendar-based fertility awareness method that estimates fertile days from past cycle lengths (shortest cycle minus 18 to longest cycle minus 11, or days 8 to 19 in the Standard Days version for 26 to 32 day cycles); because ovulation shifts and only about 30% of women have their fertile window within days 10 to 17, it has the highest typical-use pregnancy rate of the fertility awareness methods, roughly 12 to 24 per 100 women per year.

The rhythm method is the original calendar-based fertility awareness method, developed in the 1930s. It estimates your fertile days from the length of your past cycles rather than from anything your body is doing this cycle.

The formula

Using your last 6 to 12 cycles:

• First fertile day = your shortest cycle minus 18
• Last fertile day = your longest cycle minus 11

With cycles of 27 to 31 days, that makes days 9 to 20 fertile. The simpler Standard Days Method fixes the window at days 8 to 19 and is meant only for cycles of 26 to 32 days.

Why it is the weakest method

Ovulation moves. Only about 30% of women have their fertile window entirely within days 10 to 17, and stress, illness, travel, or a short night can shift it in any given cycle. Because the rhythm method cannot see that shift, calendar methods have typical-use pregnancy rates of roughly 12 to 24 per 100 women per year - the highest of any fertility awareness method.

What to use instead

For conceiving, a calendar estimate is a fine starting point, but pair it with cervical mucus and an ovulation test. For avoiding pregnancy, use the symptothermal method, which reads real-time signs. The ovulation calculator and fertile window calculator do the calendar maths.

Full context in the beginner's guide to fertility awareness methods.

Femora predicts your window from your own history, then lets you confirm it with the signs that matter.