I was working 16 hours a day up until I was 8.5 months, in a physically very demanding job, so probably not the best person to ask! LOL And I didn't have SPD.

What is your posture like? Maybe have a look at some kind of belly belt, to help. But most of all, listen to your body!!
If you are having issues, go and talk to your doctor about it and get some help and slow down where and when you can.