I suspect it's more a matter of ejecting poorly implanted zygotes—implanted enough to get some nutrients, but not enough to overtake the mother's hormonal balan

I suspect it's more a matter of ejecting poorly implanted zygotes—implanted enough to get some nutrients, but not enough to overtake the mother's hormonal balance. A poor implant could grow for a period of time before ultimately failing, fomenting a much more dangerous (for the mother) situation later in the term.