If you worry because you can't quite feel the little nugget. Drink some juice and lay down on your back. Place your hands on the uterus and with a little pressure not much you should feel them. Don't let the man child or teenagers try this with cold hands ... you might feel his or her revolting after they feel them move. Lil tried to escape my uterus the hard way after that. Fun times
I'll be 21 weeks on Sunday and I first felt my little one move at 18 weeks. I hardly feel her move now but its normal. My first one I didn't feel any movement until 22 weeks, so you should be feeling something soon, as long as the doctor said your little one is doing great. Try not to worry, its normal!