Although large numbers of pregnant women get back pain during pregnancy, there are some things that can be done to reduce the risk.
Lying down can be very uncomfortable and can lead to back pain during pregnancy. Finding a comfortable position is essential and can sometimes be achieved by lying on your side and using pillows under the tummy for support.
Lying with the legs supported by pillows (see below) may be good to try in the early stages of pregnancy but should be avoided in later stages as the pressure of the baby resting on major blood vessels in this position may be harmful.

One of the major causes of sacroiliac joint dysfunction are actions that involve one leg and not the other. An example would be getting in and out of bed.
To avoid straining the joints it is a good idea to:
More examples can be found on this pregnancy and lower back pain page.

Similar principles apply here as they would for the general, non-pregnant population. Ensure you maintain a correct sitting posture and use a lumbar roll to support the lower back when sitting.
Standing for long periods with an increase in the hollow in the lower back can cause problems. Try to avoid prolonged periods of standing if at all possible. If it can’t be avoided take frequent breaks to change the position of your spine. If you can try standing with a foot on a block and make sure that any work surfaces and units are at the right height.
Lower Back Pain Toolkit Home Page
14-Dec-2012