Because of the increase in the volume and weight of the uterus, the center of gravity of the body of the pregnant woman is shifted. As a result, the spine straightens, the lumbar lordosis deepens and a "proud posture" appears. The thorax expands, the ribs become more horizontal.
Certain changes occur in the tissue itself, which is associated with shifts in the phosphorus-calcium balance and increased blood supply to the bones. The connective tissue of articular ligaments and cartilage loosens. These changes occur under the influence of relaxin, which is formed in the placenta. Increases mobility in the sacral joints, symphysis. Pubic bones somewhat diverge (by 0.5-0.9 cm). These changes lead to a slight increase in the direct size of the pelvic inlet, which facilitates an easier passage of the fetal head through this plane during labor.