[general_re]

I'm no biomechanic either, but what I do know is that the narrowness of the female pelvis is a result of the gait changing to permit bipedalism. In order to have a reasonably smooth gait when upright, the pelvis had to narrow somewhat - compare the way chimps walk upright when they're on two feet. It's more of a "waddle" than a walk, and running upright is right out of the question for them. It's a tradeoff - they get easier births, and we gain some altitude.

The incomplete formation of a baby's skull is a partial solution to the problem - the fontanels allow the infant's skull to compress somewhat as it passes through the birth canal. Those of you who have witnessed a birth will probably have noticed that babies tend to come out pointy-headed in many cases. Another part is the hormone "relaxin", which softens and loosens the (anatomical parts alert - stop reading now) cervix and the vaginal canal, and makes it a bit more flexible and open, along with the pelvis.

Like I said, it's only a partial solution to the problem of squeezing a baby through the birth canal, though - as my wife readily assures me ;)

[general_re]

I should add, of course, that as the pelvis got narrower, brains and heads got bigger, so the problem came about from both ends. Just as the pelvises of women were getting smaller, the heads of babies were getting bigger. And the end result that birth is much, much more painful and difficult in humans than in almost any other creature - those of you who have seen a pet or other animal give birth, and a woman give birth, have probably noticed that one seems much more difficult and painful than the other.

[stanz]

Thanks for your input.
Any attempt over evolutionary time to add girth to the pelvic area would definitely compromise the ability to maintain bipedality and a steady gait.
Although the fontanelles do permit a shrinkage of the infant's skull during the birth process, they can only acccount for a variable amount. Since infants have been on the average increasing in size at birth due to such factors as better prenatal care,overall increase in the health of the mother, and an increase in overall body size from generation to generation, larger infants with increased cranial size has been the result.
I think that of the 8 lbs and 4 oz my daughter weighed at birth, it felt like the bulk of that weight was in her head.