The method relies on abstinence during the most fertile period of a woman's menstrual cycle. For a woman who has regular 28 day cycles, this is around days 10 to 17 of the cycle.
It is the only method of birth control condoned by the Catholic Church, because it doesn't interfere with conception, so allowing nature to take its course.
It is believed that the method works because it prevents conception from occurring. But says Professor Bovens, it may owe much of its success to the fact that embryos conceived on the fringes of the fertile period are less viable than those conceived towards the middle.
We don't know how much lower embryo viability is outside this fertile period, contends Professor Bovens, but we can calculate that two to three embryos will have died every time the rhythm method results in a pregnancy.
Is it not just as callous to organise your sex life to make it harder for a fertilised egg to survive, using this method, as it is to use the coil or the morning after pill, he asks?
Professor Bovens cites Randy Alcorn, a US pro-life campaigner, who has equated global oral contraceptive use to chemical abortion that is responsible for tens of thousands of deaths of embryos, or unborn children, every year.
But says Professor Bovens: if all oral contraceptive users converted to the rhythm method, then they would be effectively causing the deaths of millions of embryos.
Similarly, regular condom users, whose choice of contraception is deemed to be 95% effective in preventing pregnancy, would "cause less embryonic deaths than the rhythm method," he says.
"...the rhythm method may well be responsible for massive embryonic death, and the same logic that turned pro-lifers away from morning after pills, IUDs, and pill usage, should also make them nervous about the rhythm method," he contends.