The Unions of Elves and Men

[Mithadan]

Going back to the original question in this thread (and feel free to continue discussing Arwen, Elladan and Elrohir), Tolkien never writes about a male Elf marrying a female Man. Huinesoron mentions the following.

I vaguely recall one in-universe explanation holding that a mortal woman wouldn't be strong enough to bear a half-elven child (think the death of Miriel), but that might have been a fan-theory. Another possibility, which spans the border between internal and external, is that a woman can endure the loss of her husband because of her children, whereas fathers do not have the same attachment. (Not true, but Tolkien might have considered it to be.)

I do not recall an explicit mention that a woman cannot bear a half-elven child. However, NoME may provide a potential answer. Writing in the late 1950s through the 1960s, Tolkien discusses two aspects of Elvish child-bearing. The first is gestation. He goes back and forth a bit on this issue, stating in one place that an Elvish pregnancy lasts 9 years of the sun, then changes to one full year, and later amends to 3 years. This suggests a significant physical impact of an Elvish pregnancy. Then Tolkien mentions the recovery period after an Elvish child is born. He suggests that the mother's "time of repose" can be twelve years or more. While JRRT does not actually say so, perhaps he reasoned that a human woman simply cannot survive the effort needed to bear an Elvish child. Of course, this skirts the issue of a half-elven child...