I sort of asked my obstetrician about this - more in terms of whether because I had forceps (after an epi) the first time whether it was likely I'd need forceps again potentially for no. 2.

After he said "no" I was only half-listening (!) but he did talk about the strength of the uterus having a lot to do with it and told me that in my case I was trying to get a big baby out of a small hole but that next time it's likely to be easier as the uterus will be stronger. He also said that having an epidural doesn't really affect this because some women who've had them will end up with their uterus naturally pushing the baby to their perineum almost before they know whereas others (like me) can push for 2.5 hours with nothing doing.

He also talked about the relationship between a woman's shoe size and the baby's birth weight. Again, I was only half-listening (sorry this was after being in labour for 3ish days so mind a bit fuzzy) and I can't remember whether he said a baby's birthweight should be around 1 pound larger than her shoe size OR whether he said that a woman should be able to comfortably birth a baby 1 pound larger than her shoe size. Slight difference in those two things but general gist is that there's a natural correlation.

Anyhoo, I have very small feet (size 5 and a half - Hoobley, this is a UK 3.5), so I 'should' have had a baby who was 6 and a half pounds. So the fact that my daughter was 8 pounds 10oz was actually very large FOR ME, though obviously not ginormous in the whole scheme of things.

I know that theory sounds very dubious but he is no crackpot!