It really depends with sizing... some people can put on a lot of extra weight in their pregnancies (like me) which changes their size completely, - whilst for others it goes straight to their belly!
If she is 12 weeks and hasnt gained much as yet, perhaps she wont put on too much 'excess' weight anyway. I'd stick to the size small's, but maybe not any smaller than that just to be safe? I would say that unless you put on more weight than average, that the maternity clothes are fine in your pre-preg size...