What sort of size are you and do you have a Bravissimo anywhere near you? They will fit you properly and guide you on size.
You probably only want 1-2 to start with. Your breasts are a bit unpredictable at this point. Bravissimo will normally size you up in the cup and have the back on its very loosest setting, so that as your rib cage (likely) shrinks, you can tighten it. I have to say that they ones I had for the first month or so would have hung off my after that, and I've a pretty ample chest anyway.
Bravado (and a number of others) do stretchy bras that aren't a specific size - I have changed size several times since DS (4.5 months) was born. Having initially been massive, I have gone down a size since my supply regulated.
There's every chance that the size you buy now won't be right once baby arrives.
Yes! The Bravado stretchy ones are amazing. You can get them in John Lewis or online from NCT store (in more colours). I could be anything between a C and an F cup depending on the day of the week when my supply was establishing itself. The Bravado was always comfy. Now I'm back down to an A/B cup and it still fits beautifully.
John Lewis were great when I went in for a fitting. Got two lovely nursing bras for £30. I didn't bother getting them for the first four weeks while my breasts were still adjusting to the milk supply, so I just wore maternity bras until then.
Must say it's much easier now that I wear nursing bras, especially at night. They are well fitted so there is no more leaking all over the bed and my baby
don't go to M&S or Mothercare for fitting, they will stick you in wildly wrong sizes due to their out-dated measuring system. I agree, get something stretchy (bravado or for a little more support, Royce Blossom) and then measure yourself according to instructions here: thebetterbracampaign.blogspot.fr/p/fitting-advice.html I personally hated the non-wired nursing bras with passion, so bought normal underwired ones and converted them myself.