During my first trimester, I thought I might get away with continuing to wear my low-slung hipster jeans for the duration. And I probably could have eeked them out for way longer than I did, had I invested in button extenders or a Bump Bandeau (as recommended by JTryner in an earlier thread). But there was always going to come a point when I got a little larger all over (be it from swelling, fat or, y'know, the extra 2 ½ pints of blood that swishes around in women when they're growing a foetus).
Generally, you should buy maternity clothes in your pre-pregnancy size. As maternity clothing provides extra room around the belly and bust, it gives you the comfort you need for your changing body. If you're in between sizes or are still unsure of what size to buy, consult our maternity wear sizing chart. Take measurements around the fullest part of your bust, your hips (under your bump), and your bump circumference, and compare these numbers to the sizing chart to find your ideal fit.