" One major drawback with being bipedal is coping with pregnancy. Pregnant mammals, four-legged or two-legged, have to carry a lot of extra weight not only from the fetus but also from the placenta and extra fluids. By full term, a pregnant human mother’s weight increases by as much as 7 kilograms (15 pounds). But "

― Daniel E. Lieberman , The Story of the Human Body: Evolution, Health, and Disease