Funnily enough I read this passage the other day, and spent about 30 sec on wondering about the obstetric question, and another 30 sec on the literary/theological question. So thanks for setting me thinking more deeply.
Unfortunately the last twins I delivered were in 1979, and definitely didn’t include prolapse of a hand. But I can remember some of the wide range of malpresentations twin pregnancies allow, and would hesitate to exclude possibilities based on more common outcomes. I can imagine such an unusual situation being reported in a letter to the BMJ by some country doctor a century ago as “Remarkable case of hand presentation in twin pregnancy.” Weird stuff happens in obstetrics, and an awful lot of babies have been born over the millennia.
Not surprisingly this passage has been dealt with by interested medics - I found one article by a midwife, and this more detailed exploration in the Tyndale Bulletin from 2017, which may be of interest.
But aside from the obstetrics, it’s important to ask why the story is there in Genesis at all. If such a unique birth did occur, it would certainly have been worthy of recording in a family tradition in its own right - but scarcely worth inventing out of whole-cloth for the “final” account in the Torah, since neither Perez nor Zerah play any further part in Genesis.
The episode has some intrinsic importance in establishing Perez as the firstborn of Judah - but that would not normally have been in doubt, unless there was something confusing about the birth of the twins. And, as I said, that progeniture is of minor concern in Genesis.
In the context of Genesis, the parallels with Esau and Jacob as a “conflicted” twin pregnancy might have been of some interest : perhaps the Lion of Judah takes after his father Israel. But again, I doubt one would make up such an account to establish a rather nebulous parallel.
In the broader history of Israel, Zerah is the ancestor of Achan, who “brings trouble on Israel” by stealing devoted items from Jericho, but if you refer that back to this birth account, you have to say “So what?”
More potentially significant is Perez as the ancestor of King David (and of Jesus the Messiah), both of whom have genealogies full of dubious events that would be expected to exclude them from royal honour.God chooses David as his “firstborn” though he is youngest son, and his genealogy reflects that in numerous ways, such as his descent from the Canaanite Rahab and from the foreigner Ruth.
So Perez’s “younger displaces the older” theme, like Jacob’s with Esau, forms a part of the pattern of the royal narrative within the Old Testament as a whole; but has little obvious purpose in Genesis itself. Neither seem to me very good reasons for making up an impossible birth and, more to the point, including it in a highly literary book in which words are never wasted - Genesis is not the letters page of the BMJ.
For those reasons, I’m unwilling to dismiss the essential factuality of the account, whilst agreeing that it would be a highly unusual event. If it were not, it would have been buried in history anyway.