Women's backs are built for pregancy (but still ache anyway)

A scientific comparison between of the lower backs of women and men has shown that women's backs are specially built for carrying the extra weight of pregnancy. Similar studies on our evolutionary ancestors bore the same result.

When human ancestors changed from walking on four legs to two, the bones and muscles of their spines had to evolve to accommodate the shift in weight. The lower spine curved, to shift the shoulders back, and keep our center of gravity over our two legs.

The term center of gravity refers to the mathematical average location of an object's weight. When your the center of gravity extends in front of the end of your feet, you fall over. You can experience this for yourself. Stand keeping the back of your feet and hips against a wall, and move your center of gravity forward by bending at the waist. If you do this, you'll know when your center of gravity is beyond the end of your toes.

Because of the extra weight of pregnancy, women's lower backs evolved differently than men's. The curve of a women's lower back spans three vertebrae; in men, it spans just two. The added vertebra helps distribute the pregnancy weight over a wider area. The joints located behind the spinal cord, called zygapophyseal joints, are 14 percent larger in women than in men, suggesting that the joints can bare more weight. These joints are also oriented at a slightly different angle in women, allowing them to better brace the vertebrae against slipping. link