I went into Mothercare to be measured and the woman was rubbish - I'm usually a 34D and she put me in a 38B which was completely wrong for my shape and didn't fit at all - when I questiond her, she spent 5 mins telling me how well it fitted! I went to Mamas and Papas instead, and was measured 36DD - my underwires were cutting into me, so I knew I needed to go up a cup size, and wanted to be able to close the maternity bra tighter at the back, as poster above said you need to be able to close it on the tightest fitting so you can expand (otherwise 34 would've been fine for a while longer) - no point buying a bra that will be tight in a few weeks. My tip is definately trial and error - unless you have gone much bigger, try the next cup and back size up and see where you go from there.