I've been noticing how large my uterus is (from early on, like 8 weeks) this time around. It is my third pregnancy, so that might have something to do with it, but right now at 13 weeks my fundus (originally felt by me and confirmed by my midwife) is about 2 fingerwidths below my navel. I don't think it was that high with my previous ones until around 20 weeks. My midwife was asking how sure I am about my dates, and I am 100% sure of when this baby was conceived, so it is not a miscalculation of dates.
I wasn't originally going to get an ultrasound this pregnancy (didn't with my last either), but if this continues until the point where you'd normally do an anatomy scan, 20ish weeks, (since I know that fundal height isn't always reliable until then) I think I might just to rule out twins or any issues.
My mind has been trying to figure out what it could be other than twins and I've come up with - lots of fluid (but that would be one hell of a lot of fluid at this point), fibroids (which run in my family but I personally have no history of), or just it being my 3rd pregnancy.