What is the normal sizing of maternity clothes?

Loose is best. This is a time to go for comfort and function over fashion. Pregnant women should look for oversized clothing they already have or can borrow. They can also buy boys' shirts. There is little need to spend a lot of money on fashionable clothes for pregnancy. A couple of pairs of pants and skirts with expandable waist might be enough. If planning to breast feed, pretty but functional tops are good.
Maternity. I am sorry, but this not a medical question to answer. A maternity store would be the right place to ask this question.