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. What a load of BS! Talk about a missuse of one's science posture ...
I agree with you that this is a load of BS, but only because IMO it doesn't matter in the slightest. His technical point is that an egg which is fertilized earlier or later than the optimal time is much less likely to survive. Any IVF clinic knows this well -- there's a very narrow window in which an egg can be fertilized and become a viable embryo, but for a time before or after that the egg will still fertilize but won't get past one or two divisions before dying. Obviously the clinics choose to ensure that fertilization happens at the optimal time whenever possible; what the guy in this article is pointing out is that the rhythm method involves choosing NOT to fertilize at the right time, even though this doesn't consistently equate to not fertilizing at all. Eggs have to be perfectly ripe to fertilize AND continue developing, but they don't have to be perfectly ripe to just fertilize and form a zygote.