The NewScientist brings us news that Rhythm method criticised as a killer of embryos.

Isn't it ironic? Like a fly in your Chardonnay?

The range of birth control choices may have become narrower for couples that believe the sanctity of life begins when sperm meets egg. The rhythm method, a philosopher claims, may compromise millions of embryos.

"Even a policy of practising condom usage and having an abortion in case of failure would cause less embryonic deaths than the rhythm method," writes Luc Bovens, of the London School of Economics, in the Journal of Medical Ethics.
...
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.

So, the rhythm method, despite being ineffective (at least as compared to real methods of birth control), also might not accomplish the goal the Catholics hoped for, instead the opposite may occur.

The problem with this article, as much as I enjoy the schadenfreude it entails, is that it was written by an economist (in other words a non-scientist and self-identified purveyor of bullshit), and has lots of assumptions. The paper can be read here and PZ Myers' comments here. PZ thinks the findings are "obvious" however I don't know enough about reproductive science to evaluate this paper, there are virtually no references.