[austinjp]

That's not my understanding of the modern interpretation of head size and pelvis size. I believe current thinking is that the (healthy) mother simply cannot metabolise nutrients efficiently enough to sustain further growth of the (healthy) foetus beyond a certain size. Evolutionarily, the pelvis has adapted as far as necessary, and no further.

[austinjp]

Although actually your statement and mine aren't incompatible.

Anyway, it's quite a fascinating area of study, including biomechanics, anthropology, metabolism, and more: https://en.m.wikipedia.org/wiki/Obstetrical_dilemma