I was a 14 (16 depending on the fit) and I started wearing maternity clothes in my third or fourth month. But it was more for comfort. No one could look at me and tell I was pregnant until maybe 6 or 7 months. At one point I had to let people just feel my stomach (b/c it had started to harden) b/c they didn't believe me.
Well I was a 14/16 prepregnancy. Im pear shaped and have always had a smaller waist even at my heaviest. Even with that, I didnt really show until 20 weeks. I mean, naked or in a snug shirt I showed at about 16-17 weeks but in work clothes or sweaters people didnt know until 20-22 weeks. That being said I was really "wrapped around" with my dd. I never really popped out front even at the end.
Re: If you were a size 16-18 pre-pregnancy....