Multiple examples exist in Tolkien’s writings of children being born to a human father and an elven (or at least mostly elven) mother. However, we see no cases of children being born to parents where the mother is human and the father is an elf. It may be that the scenario of such a pair attempting to beget a child simply did not occur. Regardless, I still find myself wanting to investigate whether such a union could produce healthy offspring, as I suspect the answer may be no.
In Tolkien’s “Laws and Customs Among the Eldar”, it states, “As for the begetting and bearing of children: a year passes between the begetting and the birth of an elf-child, so that the days of both are the same or nearly so…” Human gestation period is considered to be 40 weeks – a little more than 12 weeks less than the gestation period of elves.
As I understand it, half-elves, at least in their youth, more closely resemble elves than humans in their physical characteristics. Because they are given the choice of their doom, their bodies must be formed in a way that allows them to withstand thousands of years, as an elf’s body does. Only if the half-elf chooses mortality, it seems, does a change occur that causes them to age more in the manner of humans and eventually die.
Due to this, it can likely be assumed that the prenatal development of half-elves is more similar to that of elves than of humans. And therefore, it seems unlikely that a human female would be able to carry such a baby to full term (because while she might bear it full term by human standards, it would not be full term for an elf or half-elf). Whether or not the preterm child would be able to survive, considering the limited medical technology in Middle-earth, is debatable.
We should also look at the effects bearing a half-elven baby would have on a human woman. “Also the Eldar say that in the begetting, and still more in the bearing of children, greater share and strength of their being, in mind and in body, goes forth than in the making of mortal children” (Tolkien, LaCE). Pregnancy with a half-elven baby would be even more taxing on a human woman’s body than pregnancy with a human child. While an elven female’s body is naturally able to withstand the higher demands of such a pregnancy, a human female may not be capable of it. Or, if she is capable of it, it may be damaging to her.
And now for a look at the spiritual side of things. “For all the Eldar, being aware of it in themselves, spoke of the passing of much strength, both of mind and of body, into their children, in bearing and begetting. Therefore they hold that the fëa, though unbegotten, draws nourishment from the parents before the birth of the child: directly from the fëa of the mother while she bears and nourishes the hrondo, and mediately but equally from the father, whose fëa is bound in union with the mother’s and supports it” (Tolkien, LaCE). Whatever the nature of this spiritual “nourishment” the child draws from its parents during pregnancy, it is clear that elves (and therefore, likely also half-elves) require more of it than humans (if humans require it at all – it is not specified). Is a human woman capable of providing this spiritual nourishment? Is the bond of union between a wedded male elf and human female the same or similar enough to the bond between wedded elves to allow the indirect spiritual nourishment from the father? Considering the fact that female elves seem to have no difficulty bearing half-elven children, I think it likely that the answers to these questions is yes. If the bond between a wedded human and elf were not the same or similar enough to the bond between wedded elves, or if a human was incapable of providing the required spiritual nourishment, it would undoubtedly cause the elven mother of a half-elf difficulty in her pregnancy. No such difficulty is noted. However, it may be that humans cannot provide as much spiritual nourishment, in which case, the elven parent would have to provide more than he or she would if paired with another elf.
In conclusion, while it seems definitely possible for a human female to beget and bear a half-elven child, the risks for both mother and child would be significantly higher than for a human female to bear a human child or for an elven female to bear a half-elven child.