As you may know your hormones affect the muscles and ligaments in preparation for delivery. They become more lax and open which allows the bones to shift throughout the body. When this begins to happen there is a greater strain placed on the muscles causing muscle spasms. Because of this, many times the muscles then compress the nerves running through them. During pregnancy this frequently manifests as sciatica, carpal tunnel or thoracic outlet syndrome. As the pregnancy continues to develop your center of gravity shifts forward and your muscles must continually compensate for this change in their normal functioning.