Annie, I can feel my fundus (the top of my uterus) easily, and my uterus is very pronounced. I could feel my uterus right after it popped over my pubic bone, at about 12 weeks. And I've been able to measure growth every week since then. I am now 16 weeks, and I think I've been able to see my uterus when I lay flat on my back for a week or maybe two, but probably not more.
BUT I could not identify my fundus, at all, not at all, when I was pregnant with DD, and I was definitely thinner then. I could not even find it after I gave birth and the nurse was like, "Keep checking your fundus to make sure your uterus is getting smaller." So I think being able to find it this time is some sort of fluke, or maybe just because I was poking at my tummy on a daily basis even before my uterus popped out. I guess I'm trying to say that you shouldn't worry too much if you can't find it.
I've been freaking out in anticipation of my next appointment, on Wednesday. I was fine, smooth sailing, etc., until Friday when the office called to remind me of my appointment. Suddenly, I'm in a panic. I even had a dream last night that I was bleeding.