Can't feel uterus...

I know I am probably just working myself up but it would help to hear from women thy have had multiple pregnancies. I am 12 + 2 with my 4th child. I went in and had a sonogram and heard the heartbeat (168) at 8 + 5. I'm just so worried that when I go in for my next appointment they won't find a heartbeat. I have been trying to feel for my uterus but I haven't felt anything. I remember being able to feel it with my last pregnancy at 13 weeks. I have gained a few pounds since then but with my fourth pregnancy shouldn't I feel my uterus by now?