Abdominal...hardening?

[sarapod.livejournal.com]

Hello all.

I'm a little over fifteen weeks pregnant (So, about four and a half months). I'm not showing a lot yet, but you can tell that my abdomen is taking somewhat of a more "roundish" shape. This is kind of a weird question, but...When I lay on my back and prod around where my uterus should be by now, I don't feel anything firm (You know, like a pregnant belly, it's firm?). I'm sure this is somewhat normal - I had both a doctor checkup and ultrasound this last week and the doc seemed to find it just fine. I'm assuming the issue is because the fetus is still too small, and all the amniotic fluid provides cushion, so it feels soft when I prod at it. When should things start to "harden" up?

Thanks in advance! :)