Add to this the hormonal effects that are taking place at the same time. These hormones necessarily relax the muscles, tendons and ligaments and increase joint laxity of the female pelvis so as to allow it to become more flexible and expand to accommodate the eventual delivery of the baby through the birth canal. Thus the normally rigid lumbar spine looses some of its ability to withstand the increased shear forces and stresses placed upon it by the ever increasingly distorted posture of the woman and weight of the baby. What's even worse is that the problems don't end there. In the normal non-pregnant woman, these joints are tightly apposed and held firm by their overlying ligaments.
Little movement is possible.
However again, because of the same circulating hormones mentioned above, these joints become more flexible and unstable. Thus pain from these joints is common and frequently aggravated by prolonged standing or walking. Long car rides, with its continuous low frequency vibrations, can also produce symptoms. Pain from the SI joints will also cause frequent difficulty when rolling over in bed or trying to arise in the morning. Many women will complain of extreme low back stiffness and discomfort as other muscles of the body try to compensate for the increased laxity of the sacroiliac joints.