1. Weight gain is one possible cuplrit of back pain given that the mother's center of gravity shifts during her pregnancy. When she leans back slightly to compensate for all of the extra weight in front, it puts more strain on her lower back muscles. This causes her back muscles to have to work harder as she leans back to compensate for the extra weight.
2. The two joints between her pelvis and sacrum, (known as the sacroiliac joints), also stretch as the mother's pelvis expands for the growing baby. The stretch and pulling of these joints is a common cause of back pain during pregnancy.
3. Some babies may favor a posterior (or face up) position in the pelvis, which puts more weight and pressure on her back, causing back pain. If the baby's head hits the sciatic nerve, she can also have shooting pain across her back and down her legs.
4. A posterior baby is likely to cause back pain during labor, which is known as "back labor". Some women who have back labor can experience back pain in between contractions as well as during contractions.