**2. Physiological changes in pregnancy affecting pain**

Pain occurring in pregnancy could be a result of mechanical and/or biochemical changes arising from changing physiology. The average pregnant woman gains 10–18 kg of weight (an approximate of 20% increase from baseline), doubling the mechanical load on axial joints and ligaments [3, 4]. The core muscles responsible for core stabilization and balance are stressed as well. The gravid uterus stretches the abdominal muscles and pelvic floor muscles. There is an upward shift of the center of gravity leading to some compensatory hyperlordosis with stretching of lower back muscles, significant anterior pelvic tilt with rotation of the pelvis on the femur, and increased use of hip extensors and abductors. There is also more head flexion and drooping of the shoulders [5]. Enlarged breasts and malposition during breastfeeding could also lead to thoracic kyphosis.

Hormonally induced structural changes include increased ligament and joint laxity, decreased bone density and weaker collagen, all of which have been associated with back pain [6, 7]. The Relaxin hormone is secreted from the placental decidua and corpus luteum to increase the myometrium relaxation and cervical softening by altering the matrix metalloproteinase and glycosaminoglycan compositions [8]. While the correlation of Relaxin hormone level with pain has been inconsistent, it can contribute to joint and ligament laxity and symphysis pubis dilation [7]. There is fluid gain of 2–3 l, which are locally entrapped in legs and ankles which in addition to global water retention contributes to joint stress [9]. This further contributes to joint stress. Improper joint loading can persist in the postpartum period due to continued ligament laxity and core muscle weakness. The net effect is low back pain (LBP) and pelvic girdle pain (PGP).
