And for 50 to 70 percent of pregnant women, back pain is just as unwelcoming and frustrating. And, contrary to popular thought, there doesn't seem to be a connection between the amount of weight gained during pregnancy and an increased risk of back pain. It usually develops between the fifth and seventh month and gradually eases before delivery. Most women have pain in the sacroiliac joint, the area where the pelvis attaches to the low back. The pain generally makes women feel stiff in the morning and then progresses to soreness.
Many theories exist to explain why pregnant women have aching backs.
One thought is that hormonal changes cause ligaments of the back and pelvis to soften, resulting in added stress to those areas and strain to muscles. A pregnant woman's center of gravity shifts forward under the baby's weight. She usually arches her back to accommodate the extra weight. Ironically, as the baby grows, the pain dissipates. Sciatica often is caused by increased fluid retention around the muscles and pelvic pressure on the nerve.