The common biomechanical cause is maximally stressed lumbar spine due to
the enlarging uterus and growth of the fetus. This increases the abdominal
sagital diameter, causing a clear shift in the maternal center of gravity anteriorly
which moves the pelvis around a new fulcrum. This shift results in direct transfer
of force to the lumbar spine resulting in stress in the lower back and pelvic girdle.
To compensate for this anterior shift, postural changes such as lordosis and inward
curvature of the spine occur which further aggravate the stress on the lower back.
reference) Musculoskeletal symptoms orthopaedic complications in pregnancy
: pathophysiology, diagnostic approaches and modern management.
Postgrad Med J. 2014; 90:450-460