I'm working on an article on SPD for OHbaby! atm, my poor boss has it bad
I had it so bad with Maya that I ended up on crutches. Didn't get it anywhere near as bad with the gremlins which surprised me given I was much bigger.
Things that help: lying on your side with a pillow between your knees, warm baths, support belts or tubigrips from the physio, and a plastic bag on the seat of the car to enable you to slide in and out more easily.
Things that don't help: Sitting for long periods of time, sitting with your legs apart, squatting, any position which opens up your pelvic joint.
It is really important to talk to your midwife about it because if you have it, it can affect the way you birth. I wasn't allowed to squat to deliver Maya because of the risk of permanent damage to the joints.
Like Maree said, the pain usually subsides post birth, altho I had a few issues with my lower back that were indirectly related to the SPD when Maya was 15 mths. It's good to talk to a physio postnatally too to make sure everything is settling down properly.