My physio told me not to buy them and to buy a belt (can't remember price but it's a lot cheaper than the shorts) or tubigrip (around $15 instead). She said the shorts stretch so once you've had the baby, you can no longer use them as they're not snug. Same applies to the tubigrip but it doesn't really matter so much at $15.

Hopefully, your SPD will go away after the birth but not everyone's does.
thanks for the feedback - i have a sacro/pelvic belt already which helps when im up and about but i can't sleep with it as its too uncomfortable