hmm, well i honestly would go by what your doctor and original us tech said, and you can mention it to your doctor and tell him/her that your concerned now. They will go about measuring and maybe another u/s to tell!
they said the baby is looking great during my regular u/s. and when I went in at 17 weeks i was measuring 17. the 3d/4d u/s tech made a comment that my uterus hadn't grown a whole lot yet and i was worried that might be a bad sign....? the baby was weighing in at 8oz at 17 weeks as well.
So did they specify if the baby it's self was growing? This would definately be something to call your doctor about. Your uterus should be measuring at 19 ish for how far along you are. Was this your doctor that said this, or the 3d/4d tech who did? I'm a little confused there.
either way i'd ask your doctor. I think if you uterus wasn't growing your doctor would be telling you at the appts.