It turns out, however, that even the rhythm method may have this effect. While the discussion is more one of medical ethics based on known statistics about the rhythm method and other contraceptive methods, rather than new experimental evidence the bottom line is that:
Given certain plausible empirical assumptions, the rhythm method may well be responsible for a much higher number of embryonic deaths than some other contraceptive techniques.
Only one assumption not supported or disproven by empirical evidence matters.
The result really depends on the simple assumption that embryos conceived outside the HF [high fertility] period are less viable than embryos conceived during the HF period[.]
All we know now is that the rhythm method makes pregnancies outside the high fertility method less likely. It doesn't tell us if that is due to differing degrees of viability, few conceptions, or a combination of the two factors. But, given the fact that oral contraceptive basically manipulate hormones in a way not all that different from the way that the body does anyway, the notion that viability might vary between high fertility and low fertility periods is very plausible.
Why does it matter? Because, if differing degrees of viability are one reason for the rhythm method's effectiveness, then there is really no moral difference between IUDs, oral contraceptives (including emergency contraception) and the rhythm method from the point of view of those opposed to the former as a form of abortion.
This effectively means that, because we don't know for sure why the rhythm method works, the only way those opposed to the other contraceptive methods can really be morally consistent is to disavow all known methods of contraception and either have sex and try to have children (or at least, not try not to have children), or to not have sex. But, celibacy is standard that historically has been reserved only for the clergy in Roman Catholicism, has been reserved only for Bishops in Orthodox Christian tradition, and has been considered unrealistic for the laity or any class of clergy, in the Protestant Christian, Jewish and Islamic traditions.
Historically, just about everyone took the former path. Women had large numbers of children, and most of those children died before reaching adulthood, thus keeping the world's population in check. Now, very few people choose that approach and almost all children who are really viable at birth live to adulthood.
About 98.8% of births are of first, second, third or fourth children (Table 79), and more than 95% of births are first, second or third children. Just 0.3% of births are eighth or later births. But, in the complete absence of efforts to avoid conception, seven or more children per woman would be the lifetime median (something seen in a few of the least developed countries in the world still today), rather that the U.S. median right now of about 2.1 children per woman per lifetime. With all due respect, most people don't see places like Rwanda as a positive model for modern family life, and even fewer see that lifestyle as an urgent moral imperative.
Almost everyone reaches adulthood. About 98.4% of boys and 98.9% of girls who are born survive to age twenty (Table 97). A majority of the 13 and a half babies per thousand who are born who don't reach adulthood, die in the first twelve months of life, and about a third of those who don't reach adulthood die in the first four weeks after their births (Table 104). Being a kid has never been safer or healthier.
The alternative to following the anti-contraception philosophy to its logical conclusion, of course, it to reject the claim that a fertilized egg is the moral equivalent of a viable human child who has been born. This doesn't necessarily imply a pro-choice position. It is possible to make a morally significant distinction between contraception methods like IUDs, oral contraceptives and the rhythm method on one hand, and surgical or chemical abortions at a stage of development when it is, at least, possible to know that one is pregnant with some degree of certainty.
If one is willing to accept that distinction, as the vast majority of Americans do, then, from a policy perspective, a focus on preventing unwanted pregnancies through contraception, preventing STDs through appropriate measures ranging from vaccinations to barrier method contraception, and preventing unwanted sex is the first priority, and the abortion issue itself is secondary.
Roman Catholic doctrine may not change as a result, but those who base their attitudes on contraception on reasons similar to those of the Catholic Church, rather than simply doing so because the church says so, need to take a hard look at whether this really makes sense.
Hat Tip to Daily Kos diarist Stagarite