The belt doesn't just support the bump it actually goes underneath the bump and is pulled tight across the hips holding all the joints together and stops the pubic bone grinding up and down when you walk. Best advice is to get the large tubigrip as others have said as you can double it up and where it as low as you want to suit yourself. A tip for after the birth is to where really tight jeans as it gives good support.
Even if you buy a belt yourself, still get a referral to a physio for some expert advice.
There are lots of things you should avoid doing which can exacerbate the condition e.g. anything that involves spreading the legs!
A good friend of mine had to stand up all through her labour and was only allowed to lay down if she had to for the actual delivery as her spd was so bad.
You may also need to be seen after the birth as well, to check all is well and they will advise on what size belt and how to wear it (i.e. what accessories it goes with....only joking about the accessories)
Gosh it is an arse of a thing, known other women who've suffered terribly from it. I know it isn't too bad but I realise getting it earlier it may be worse than last time and may last after birth perhaps. Actually I saw an osteopath last time randomly right at end and she was totally amazing and did help.
So have bought the belt and made appoint. for referral if all too complicated or will take an age i'll just find another osteo where we live now.
Hope you feel better or go into early labour ffrosty!!!