Hi there.
I had crippling SPD in my first pregnancy, Started at 20 weeks and was so bad that by 34 weeks I couldn't walk at all .
With my second pregnancy I started to feel the familiar pains by about 12-14 weeks. Got a referral to a women's health physio at my local hospital. She gave my a set of exercises to do religiously three times a day. They were pretty much pilates-based, really - lots of pelvic floor / core stability stuff, very simple to do. It didn't stop the SPD developing, but it was nowhere near as bad as first time around. In fact, I'd say from about 26 weeks-ish onwards the SPD got very slightly better.
I also put on less weight with this pregnancy (4 stone in first pregnancy , only two stone this time) and I think that did make a lot of difference, too, to be honest.
My physio also advised me that keeping a fine balance between not overdoing it and not just lying around all day was the key. So - no heavy lifting, no long shopping trips etc, no prolonged periods of physical activity. However, several bursts of short walking each day were encouraged, as sitting down a lot can really exacerbate SPD.
Ice packs down your pants can really help, too, if the pain is at the front of your pelvis