Sentence:  The Roman Catholic Church sanctions only abstention or the <WSD>rhythm</WSD> method , also known as the use of the infertile or safe period . 
Meaning of the word marked with <WSD> tags:  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)