WordNet (r) 3.0 (2006):

rhythm method of birth control
    n 1: 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) [syn: rhythm method of birth control,
         rhythm method, rhythm, calendar method of birth
         control, calendar method]