12 weeks to go? poor thing. I'm sure you've heard all the tips about how to minimse the pain/effects of spd, but just in case there's one you haven't:
-sleep with pillow or two between your knees to align pelvis, and possibly one under the bump when it gets really big, experiment until you feel supported and not in pain.
-keep legs together getting into bed/the car.
-wear silky/satin nightwear or sheets to make it easier to turn over in bed.
-try to avoid squatting or twisting positions, and getting down on the floor.
-take it easy and just because it feels better some days do NOT be tempted to go on a long walk or do excessive amounts of housecleaning/shopping (tempting when you start nesting but you will regret it!)
-wear the flattest most supportive shoes ever.
-try to sit somewhere supportive at home which lets your pelvis sit straight with no twisting, and isn't so low that you're squatting to get up. Assemble your remote, drink, books, phone etc near the chair so you aren't constantly getting up.
-if your chair at work isn't adjustable and comfortable, ask for a pregnancy desk assessment and get the chair replaced with something comfortable.
-be careful about bouncing on a gym/birthing ball. In most cases they're a good idea but squatting/jarring your pelvis isn't!
-be careful getting in and out of the bath, which isn't easy as you have to raise your legs independently, get non slip mats and hold on to something tightly.
-Don't raise your legs in the shower to shave them, honestly I've knackered my spd so many times doing this, better to stretch in the bath and let the water support you a bit.
-once it's irritated and painful, you MUST REST for it to get better, and that means no hoovering, no ironing (both jerky twisty motions) just sitting/lying down, with some very gentle pottering if you must.
-it's a good idea to have a phone near the bed if your spd is bad- I was stuck in bed with a full bladder yesterday and couldn't turn over so couldn't get out for nearly an hour. Finally managed to get out and reach the loo in time but made me aware of how long I would have been stuck alone on my back if I hadn't have had a phone next to the bed.
-let your partner read about it, sometimes they don't take pregnancy symptoms seriously unless they hear them coming out of the mouth of a midwife but unfortunately it's likely to get worse as time goes on and you may rely on him more for silly things like putting your shoes on or holding your hips steady and turning you over in bed. It's good if they know it's a real condition and not just "achy hips"
-make sure you have it written somewhere in your antenatal notes that you have spd, I always worry that if I go into labour and suddenly announce I have spd with nothing on paper to back it up, it might be ignored, and as you've probably read, labour can aggravate spd a LOT, especially if you're on your back with legs in stirrups, anaesthetised so you don't feel the pain of spd until the next day. They say to tie a piece of string around your knees to the limits of how far you can open them without pain, and keep the string on in labour so they can't argue!
I've had it since about 11 weeks so I know how you feel, my advice would be to keep nagging your midwives and get a fembrace (hip brace/support grament) or a referral to a chiropractor. I didn't do that and so I'm still suffering only so much worse 27 weeks later, and I can't leave the house by myself.
Hope this helps! x