Formal study of the incidence of low-back pain in pregnancy has been very limited. Pain can begin before week 12 and continue up to 6 months postpartum. Prepregnancy back pain and multiparity seem to be risk factors, whereas age, height, weight, race, fetal weight, and socioeconomic status do not seem to correlate. Sacroiliac pain is felt distal and lateral to the lumbar spine near the posterior superior iliac spine, and may radiate to the posterolateral thigh, usually to the level of the knee and rarely to the calf. Symptoms of sacroiliac joint pain typically continue several months after delivery.
Nocturnal pain occurs in the low back only at night while recumbent.
Understanding the normal musculoskeletal changes that occur during pregnancy is useful for targeting and treating the sites of a patient's back pain. Lumbar pain during pregnancy can stem from multiple sites, most commonly the facet joints, paraspinal muscles, supporting ligaments, or discogenic sources. Posture changes that occur during pregnancy help the woman maintain balance in the upright position as the fetus grows. The increasing weight is distributed primarily in the abdominal girth. After 12 weeks of pregnancy the uterus expands out of the pelvis and moves superiorly, anteriorly, and laterally.