rhythm method

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A birth-control method dependent on abstinence during the period of ovulation.

Wiktionary

1. n. A method of natural birth control that involves counting days of a woman's menstrual cycle in order to determine when sexual intercourse is most or least likely to result in pregnancy.

WordNet 3.0

1. n. natural family planning in which ovulation is assumed to occur 14 days before the onset of a period (the fertile period would be assumed to extend from day 10 through day 18 of her cycle)