Mechanical issues aside, the process of human reproduction involves a very large number of sperm meeting an egg. It would seem then that given conception can either fail before, during, or after the fertilization phase (there is no fourth option), there are only two logical causes of failure to conceive, failure to fertilize (before/during) and failure of the zygote post-fertilization (after). Which of these two options is dominant in typical failure to conceive? Asked another way, statistically speaking (as of course every case is different), for couples struggling to conceive, is the issue generally that the egg does not fertilize, or that the egg does fertilize but fails to develop, akin to a very early miscarriage?
There are infertile couples where there is no sperm, others where there are only small numbers, others where there are no eggs, and many more issues besides. If you're sweeping that away with "mechanical issues aside" what insight do you hope to gain from an answer to this question?– Kate GregoryApr 18, 2021 at 14:41
@KateGregory Great question. My hypothesis is that very early (zygote-stage) abortions are predominant among "failure to conceive" cases, and that the common perception of an inability to fertilize is incorrect. Rather, this would mean that fertilization is hard to get wrong provided the appropriate cells are available in sufficient number and proximity, but that the real struggle is sustaining to implantation and birth. This would also beg the interesting question of whether this fact is intentionally kept from struggling couples given the emotional implications of miscarriage.– TheEnvironmentalistApr 18, 2021 at 16:54