I think they say that it's ok for you to not feel consistent movement until 24 weeks. it might even be 28 weeks. but if it concerns you,c all your l&d department and they can tell you what is ok and what isn't for movement at this stage.
I felt my baby the first time at 16 weeks, just small flutters, then didn't feel ANYTHING until 20 weeks when I felt the first kicks, probably 5 - 10 good ones, then felt nothing again until 22 week, I am now 29 weeks and have been feeling constant movement for a few weeks now. All the books say its fine to not feel constant movement until 24-28 weeks I think.
If you are worried, pick up a doppler. It's the best money spent for peace of mind between visits. I have one coming in on Monday...I did a TON of research and got about 10 hits on another board from pregnant woman who own the same one and said they have been listening to baby's hearts since as early as 10 weeks! Not one bad review.
Here is the best price on it: