It says the ligaments that hold the joints in place become increasingly pliable during pregnancy. This is so the pelvis is able to widen during delivery. Because of this increased elasticity, the spine is put under a lot of strain during pregnancy. With the additional weight of the uterus, the woman's center of gravity changes. These changes can cause a lot of strain on the back, especially when standing.
Correct posture is very important.
It says the correct way to stand is with the shoulders back and the hips tucked in. When sitting, the feet should be resting slightly off the floor with legs uncrossed. When preforming physical tasks, be careful not to overexert yourself. It is important to lift items by bending the knees with the feet are apart for added support. Be sure to use the leg muscles instead of the back.