The weight of the fetus, the enlarged uterus, the placenta and the amniotic fluid, together with the increasing curvature of her back, puts a large strain on the pregnant woman's bones and muscles.
Most kinds of back pain are normal in pregnancy, but it can also be a warning sign of a kidney infection.
Progesterone also causes a loosening of ligaments and joints throughout her body. Pregnant women may be of greater risk of sprains and strains because ligaments are looser, and because their posture has changed.