As many as 80% of pregnant women will experience low back pain, especially in the their third trimester of the pregnancy.
Typically, the pain will be in the sacroiliac joints, and is probably due to a combination of carrying extra weight and hormonal factors that act to widen the sacroiliac joints and increase the size of the birth canal.
Read about Sacroiliac Joint Dysfunction (SI Joint Pain)
Physical therapy can help reduce the symptoms of low back pain. Medications, which may potentially harm the fetus, should be avoided. The back pain usually goes away after delivery.