Normal Pregnancy Changes During pregnancy, women gain an average of 11.3-15.9 kg, shift their body’s center of gravity, and experience a variety of hormonal changes and biomechanical changes that place significant stress on the medial skeleton and pelvis. Weight gain and hormonal changes during pregnancy cause musculoskeletal changes. I. Increased stress on certain joints 1. In order to compensate for the enlarged uterus and the change in body center of gravity, excessive anterior convexity of the lumbar segment of the spine, forward neck flexion, and downward movement of both shoulders usually occur. Tension, weakness and separation of the abdominal muscles further impede the maintenance of the median position and place greater tension on the paraspinal musculature. 2. Joint laxity of the anterior and posterior longitudinal ligaments of the lumbar spine can make the lumbar spine more unstable and prone to muscle strain. 3. Widening and increased mobility of the sacroiliac joint and pubic symphysis in preparation for delivery of the fetus through the birth canal. 4. Lengthening of the vagina, enlargement of the genital fissure and relaxation of the posterior vaginal wall. 5. There is a significant increase in anterior pelvic tilt with increased use of the hip extensors, hip extensors and ankle plantarflexors. In order to maintain trunk movement, the distance between the feet is widened when standing, which in turn leads to gait changes. 6. Fluid retention can cause compression of certain vulnerable structures (such as the median nerve). 7, ligamentous laxity can affect the stability of all weight-bearing joints from the pelvis to the foot. Hormonal effects 1. Many of these changes may be mediated by relaxin produced by the corpus luteum, meconium and placenta. Its concentration peaks in early gestation, when relaxin is thought to be involved in placental implantation and growth. During subsequent pregnancies, relaxin contributes to myometrial relaxation, separation of the pubic symphysis and cervical softening. 2. The laxity of the joints of the extremities seems to be caused by other factors. 3, Subjective joint pain is associated with elevated estradiol and progesterone levels, but not with elevated relaxin concentrations.