I'd say by weeks 14-15 I definitely started to "round out". I was wearing maternity pants by then, mainly because I started overweight and the regular pants weren't comfortable by then. I'd say my hip size definitely increased at that point, though my waist size was probably about the same.
Remember that everyone is different! Some ladies don't show until much later in their pregnancy (especially if it is their first), but I definitely embraced by bump early on in the name of comfort. Does your dress have a zipper or a tie back?