Seventy-five percent of women who are pregnant have some low back pain, he says. Women who have their babies in their thirties and forties may experience more back pain than younger women if they're not in as good shape, he says. Women can get back pain throughout their pregnancy, but some women report an increase in the last few weeks. The pain is usually in the low back and can radiate into the buttocks and the backs of the thighs. Walking, sitting, lifting and bending forward can worsen pain.
Many women experience more back pain at night.
There are a number of reasons for the pain. For one thing, the weight and size of the fetus cause a woman's center of gravity to shift forward, so there is more curvature in her lower spinal column. This causes some low back muscles to shorten. In addition, the abdominal muscles, which normally stabilize the pelvis and support the back, are stretched and weakened by the growing uterus. During pregnancy the body releases higher levels of the hormone relaxin.