Quote from : A Short History of Nearly Everything Book

Bipedalism is a demanding and risky strategy. It means refashioning the pelvis into a full load-bearing instrument. To preserve the required strength, the birth canal in the female must be comparatively narrow. This has two very significant immediate consequences and one longer-term one. First, it means a lot of pain for any birthing mother and greatly increased danger of fatality to mother and baby both. Moreover, to get the baby's head through such a tight space it must be born while it's brain is still small - and while the baby, therefore, is still helpless. This means long-term infant care, which in turn implies solid male-female bonding.