Anyone have any size tips for maternity clothes from ASOS?

I'm interested in ordering a dress from there - it is fitted and stretchy. Their measurements look closer to juniors than regular women's. I'm leaning towards an 8 (normally a 4-6 in GAP, LOFT) and I would want to wear it the first time when ~18 months so not a huge belly. Does there stuff run really small such that I should order a 10?