jo i google it and apparently its this!!
Trying to balance that weight out in front would add all kinds of strain to the back muscles. But to compensate, women's lumbar vertebrae (lower spine bones) are reinforced and slightly curved to allow them to adopt a more pronounced S-curve -- forward at the base of the spine, but curving backward as it rises. This keeps the centre of gravity over the legs and reduces back strain. The spine slowly shifts back to its original shape after pregnancy.
it goes on about it being part of evolution lol