Super mom to a 5 1/2 year old girl, almost 4 yr old girl, and 2 year old boy.
What I do is just buy one that fits and isn't too expensive, then cut it apart at the seams and lay it out on the floor. Then you can make as many duplicates as you need from the "pattern". I know it's not the technical way to do it, but I'm really bad at following patterns and if I see how it fits in front of me, it's easier to duplicate it on my own. You said you're a beginner too, so I hope this helps. Do you know the site http://www.gatheringguide.com? It lists dress shops by city so you can call around and see who has maternity dresses in stock. I'd give you specific store recommendations but I don't know where you live.