Pregnancy is the most common cause of posterior pelvic pain in women. Other causes of this lower back pain in both men and women include osteoarthritis in the back, a fracture in the lower back, and overuse or straining of the muscles or ligaments at the base of the back. Posterior pelvic pain may also be referred to as lumbar back pain, pelvic girdle pain, or, in pregnant women, pregnancy back pain.
The posterior pelvic area is composed of the lumbar or lower vertebrae, the upper part of the left and right side of the pelvis, known as the ileum, and the triangular shaped bone, called the sacrum and located at the base of the spine. This region also includes the two sacroiliac joints located on the left and right side of the body where the sacrum touches the ileum. The posterior pelvic area bears the weight of the upper body and is an anchoring foundation that allows walking, twisting, and lifting.
This pain is very common amid pregnancy for two reasons. First, as the fetus grows during pregnancy, the stomach muscles are elongated and distressed until they can no longer bear all the weight of the baby and the load is shifted onto the lower back. Second, some pregnancy hormones cause joints, including the sacroiliac joint, to slacken, allowing the pelvic region to widen in preparation for passage of the baby. This loosening of the pelvic joints makes the sacroiliac joint more prone to pulls and tears, which can produce significant posterior pelvic pain.
Another common cause of sacroiliac joint pain is osteoarthritis. Joints are blanketed with a soft rubbery substance known as cartilage. Osteoarthritis causes the cartilage cushioning the joints to wear away. The degeneration of the cartilage can cause a person afflicted with osteoarthritis to walk, twist, and lift at the hip differently to compensate for the lack of padding at the joints. Eventually this type of awkward movement can strain and damage the sacroiliac joint, causing swelling and radiating posterior pelvic pain.
As people age, the density of the bones and the flexibility of the joints decreases. As a result, the bones become frail and more apt to break, and the joints become stiff and function less effectively. Occasionally, the pain will be the result of a break in the ilium or the sacrum. Stiff joints can also tear more easily, causing significant lumbar back pain.
Overuse and straining to lift large amounts of weight can tear the sacroiliac joint or pull muscles in the lumbar region. Lifting and twisting at the waist while bearing a heavy weight are particularly stressful to the pelvic girdle. Tears of the sacroiliac joints and pulled muscle will often be the cause of posterior pelvic pain.