Pregnant information.
Why pregnant women can stand up right and not fall over?
The reason why women do not fall over is because of the way the females spine can adjust to balance the extra weight of the baby.
Females have lower backbones and hips that have changed over million of years to be different from men, so they are more flexible and give support to the baby. A female hips joints is also slightly bigger then a man's hips.
This allows the mother to carry the baby while still being upright on two legs.
When you consider that the foetus and placenta can weigh up to 16lbs/7.25 kilos and the weight is towards the front of the hips resulting in the mother leaning back.
Early evolution women would have been more designed to be hunter-gatherers and survival from predators was of utmost importance rather than today.