When do the postural changes of pregnancy occur?

Women experience a progressive increase in the anterior convex shape of the lumbar spine during pregnancy. This change, termed lordosis, helps keep the center of gravity stable and over the legs as the uterus enlarges (see below). Late in pregnancy, aching, weakness, and numbness of the arms may occur secondary to compensatory anterior positioning of the neck and hunching of the shoulders in positional response to exaggerated lordosis. These positional responses put traction on the ulnar and median nerves, resulting in the previously mentioned symptoms.

Relaxin in pregnancy is secreted by the corpus luteum, the placenta, and part of the decidual lining of the uterus. It is thought to cause remodeling of the connective tissue of the reproductive tract, especially inducing biochemical changes of the cervix. Although relaxin levels are more than 10-fold higher than nonpregnant levels, it is not currently thought that this hormone has a direct effect on actual ligamentous loosening. Joint laxity and shifting center of gravity can contribute to an increase in gait unsteadiness. These changes are most exaggerated in later pregnancy. More than 50% of gravid females complain of back pain during pregnancy, which may also be due to sacroiliac joint dysfunction or paraspinous muscle spasm.

About 4-6 per 1000 women will have scoliosis. This is not accompanied by any functional osteoporosis, although pregnancy is a very high bone turnover state, approximately equivalent to double the bone loss rates of a menopausal female, reversibly losing about 2% of bone during the first 20 weeks of gestation. Spinal changes are usually not severe enough to affect the pregnancy or the lung's functional capacity. Also, the pregnancy rarely affects the degree of lateral curvature in these cases of scoliosis. If a pregnant patient has had correction with prior Harrington distraction rod insertion, the pregnancy, labor, and delivery are not typically affected. The epidural space may be distorted, and some anesthesiologists may refuse to place epidural anesthetics in these patients.

Source: http://reference.medscape.com/