rhythm method

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A birth-control method characterized by abstinence from sexual intercourse during the period of ovulation.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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)