system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do most women's lower back sway inward?

I have always wondered why a woman's lower back dips inward towards her front? Some of them it looks over-exaggerated. Female children do not have this. 

Men for the most part do not have such a pronounced dip in their back.
gpt
The shape of the lower vertebrae is a little different as are most bones. The shape of the pelvis is very different (lower and wider) so the spine "enters" the pelvic circle at a slightly different height. The coccyx is also more movable and the sacrum is flatter and broader.  These are all almost certainly due to the shape being more conducive to pregnancy. Also, women tend to store more fat on their behinds which exaggerates the idea of a spinal curve, but it is merely the covering anatomy.