Using the rhythm method to promote fertility doesn't pose any risks.

Likewise, using the rhythm method for birth control doesn't pose any direct risks — but it doesn't offer protection from sexually transmitted infections. In addition, the risk of unintended pregnancy with the rhythm method is somewhat higher than with other methods of birth control. Although effectiveness varies, in the first year of typical use an estimated 13 to 25 out of 100 women practicing the rhythm method for birth control will get pregnant.

Dec. 17, 2011