Dictionary.net

• 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)

2006 - WordNet 3.0
By Princeton University

• 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)

2011 - English Dictionary Database
By DataStellar Co., Ltd