Reddit suggested a link similar to this one last week.

From the article:

In using the rhythm method, couples avoid pregnancy by refraining from sex during a woman’s fertile period. Perfect adherents claim it is over 90% effective – i.e. one couple in 10 will conceive in an average year. But, typically speaking, effectiveness is estimated at closer to 75%.

Now Bovens suggests that for those concerned about embryo loss, the rhythm method may be a bad idea. He argues that, because couples are having sex on the fringes of the fertile period, they are more likely to conceive embryos that are incapable of surviving.

In essence, rather than preventing fertilization, the rhythm method may be preventing implantation, thus causing more embryos than might otherwise to be rejected in preparation for menstruation. For those who believe live begins at fertilization, this could be bad news. However, since the Catholic Church does not support prophylactics, which prevent fertilzation in the first place, this news is less likely to intitiate a re-consideration of birth control than it is to simply eliminate it altogether.