I'm with StephandOwen--I bought smalls too early and grew out of them. Also, you have to keep in mind that not even all maternity wear is designed to be worn through your whole pregnancy (I know, genius right?) It is amazing how the "shirts" that went down to my mid-thigh quickly became the only ones that would cover the panel on my jeans at the end. Length on tops is crucial.
Definitely! By about 35 weeks, maybe earlier, I'm often down to a very small handful of tops that will actually cover me. It's insanely frustrating, not only because it limits your wardrobe variety, but also because by that time in the pregnancy, it's impossible to keep from spilling food all over your belly and often staining your shirt. At least, that's always been the case for me. ;)