My chiropractor recommended this if your pain is really severe: Maternity Belt
The Maternity Belt is a non-stretch back support with a fabric back panel and Velcro fastening, designed to provide support for the Sacro Iliac joint (SI joint) without putting pressure on the 'bump'.
Hormonal changes cause ligaments to become lax during pregnancy. This is normal to allow the pelvis to expand in readiness for child birth. However, because the Sacro Iliac joint is bound tightly by ligaments, the hormonal changes mean that expectant mothers commonly suffer from Sacro Iliac (SI) joint dysfunction which causes lower back pain.
As a result of the baby's growth and subsequent expansion of the mother's tummy, standard Sacro Iliac (SI) joint supports can't be worn comfortably. The Maternity Belt provides the ideal solution as it fits low around the hips, thus allowing Sacro Iliac joint support without hindering the mother and baby.
The Maternity Belt can be useful during acute and chronic cases of back pain caused by Sacro Iliac (SI) dysfunction during pregnancy. The support provided by the belt compensates for the lax ligaments and thus alleviates pain.
HTH someone, I think I'm going to get one. The chiro reckons I'm not at that point where I need to be strapped together yet, but he hasn't seen me trying to get up off of the sofa or out of bed late at night! 
I don't think it's suitable for SPD sufferers.