When a woman is pregnant, her pelvis begins to tip backwards as the baby develops, to balance the forward weight of the expanding uterus. As the pubic and tailbones move backward, the arch in the lower back increases. As the lower back arch increases, so does the curve in the upper back or thorax.
The shoulders and head move forward and the chest collapses to balance the changing weight. All these changes can increase stress on the spine, exaggerating its natural S shape. Other muscles and ligaments can also be strained.