Baby and Back
From Nature (Whitcome et al, 2007):
For bipeds, pregnancy poses a serious challenge
to already precarious balance. Anthropologists Katherine Whitcome and Liza Shapiro may have gone a long way towards finding the explanation in morphological differences between men's and women's spines. A report in Nature.com (Whitcome et al, 2007) demonstrates that the wedge-shaped lower lumbar vertebrae in women allows a bend of 28° backwards, without the shearing forces that would occur with a man's squarer vertebrae. This allowed women to adjust the center of gravity during pregnancy. The main conclusion of several commentators has been: pregnancy would be far worse for men. Heidi Ledford (2007) quotes one anthropologist as joking: "I would advise all of my male colleagues not to become pregnant."
I'm reminded of the male pregnancy hoax/art project that circulated a couple of years ago. Whitcome and her colleagues open their report with this sentence: "until recently, hominin females spent most of their adult lives either pregnant or lactating." They're right, and I probably spend too much energy pretending that it doesn't matter. I have, over the past couple of years, ended up conceptualizing the capacity to bear children as extremely costly, in an evolutionary sense, but I think that's the wrong way to look at it. It's more amazing that male and female humans were able to remain as similar as they are while maintaining this ability.