Almost all maternity web sites will tell you to wear the same size in pregnancy that you wore before. I will say something different.
For some people, this will be the case. Everyone carries differently, so there are the few who gain no weight except in their stomachs, and they can wear the same size throughout pregnancy.
HOWEVER, many women gain in other places too and do better one size up. I was a small before my first pregnancy and bought a nice selection of smalls and mediums for maternity. A lot of the smalls didn't fit toward the end. When the super-skinny saleslady was trying to convince me to buy the mediums, I was fighting her saying "No! but I am a small too!" desperately trying to maintain my image as slim, which had been difficult for me to attain. Well, she was right, and many of my customers who have a good self image already know that. Really, it doesn't matter what the tag says. You need your clothes to grow with you. Also, some tops will rise as your stomach expands and will leave some noticeable skin showing if the size is too small.
So you need to keep in mind two things: 1, your stomach will grow a lot, but 2, your shoulders will not. What I mean is that not all tops will look good one size up because they have to fit the upper part of your torso as well.
The idea is try to find appropriate tops that fit in the shoulders and still have room to expand in the stomach.
The Erin top is a great example, because the cap sleeves are sloping and therefore cal fit different sized shoulders, while the roomy fit lets it grow with your stomach.
Alas, there will always be a few things that don't quite last the whole 9 months, so do keep in mind that you may need a few extras toward the end. We have so many customers who walk in at eight months and just need something new after wearing the same thing for so long.
