Well, I absolutely did not suspect at 30 wks. It was at exactly 30 wks that my m/w palpated 2 babies, MUCH to my horror (er, surprise). I confirmed it by u/s at 32 wks. I was not bigger than I had been with my singleton, although after 30 wks I grew a lot more. I was feeling a lot of movement, but I just thought it was a very active baby. I really didn't have a lot of differences from my singleton pg until the last few weeks, and even then I was mainly just bigger. My singleton was 9-2, born at 39 wks. My twins were born at 39 wks, 5 days and weighed 7-4 & 7-6.
ETA: I'm sure a lot of it has to do with positioning of the babies. I could clearly feel baby A's head like a shelf sticking out from my lower gut, lol. I could actually hold his head in my hand. Baby B was neatly tucked behind, and quite difficult to feel. I am pg again now & dh said last night that it must be a singleton because last time he felt that whenever he touched my belly anyplace, there was always a foot or elbow sticking out.