Low back pain in pregnancy 1. What is low back pain in pregnancy? Pregnancy low back pain refers to the discomfort in the waist that occurs during pregnancy, usually in the second and third trimesters. This pain may spread into the buttocks and thighs, making activities such as walking, sitting and standing difficult. 2. Why does low back pain occur during pregnancy? 1. Increased hormone levels in pregnant women cause ligaments to relax and increase stress on the pelvis and spine.