It's believed that women died in childbirth somewhere up to 1/4 of the time for most of our history (until a few thousand years ago, since when it's slowly but greatly improved) due to our enormous heads, which would probably be the largest reason. Plus they were rather less able than normal to fend for themselves for a few months right beforehand, and were too busy (if they wanted their little bundle of genes to survive anyway) to do so for a while after.