There are two main types of back pain women experience during pregnancy with an estimated 50 to 80% of pregnant women experiencing some form of back pain.
Lumbar or lower back pain is usually experienced between 5 and 7 months although some women do experience pain as early as the second month especially in women who suffer from back pain pre-pregnancy. This type of pain is attributed to the gradual increase in weight putting pressure on your spine, postural tension as your centre of gravity shifts forward again putting strain on the spine and emotional stress which puts tension in the back muscles. This kind of pain can be relieved by maintaining a good posture, swimming or yoga, heat or ice packs, using a lumbar pillow at work or when sitting for long periods and the use of a maternity support belt which helps support your abdominal muscles and releases pressure on your lower back.
Pelvic pain is more prevalent than lumbar pain among pregnant women and is usually felt around the 4th month of pregnancy to the side and below the waistline and/or across the tailbone, the buttocks and upper part of the the thighs. This kind of pain is usually caused by the stretching of pelvic ligaments, making the pelvic muscles tense as they work to establish stability and is attributed to the release of pregnancy hormones such as estrogen and progesterone. Unfortunately this type of pain is very difficult although it is generally believed that vigorous exercise, stairs, prolonged walking or sitting and heavy lifting should be avoided.
For the best information regarding your own personal situation you should visit your doctor who may make an appointment with someone who specializes in this area.