This is my 3rd pregnancy, and at 18 weeks (with my first anterior placenta mind you) I lost movement. A quick trip to ED and I was put straight into birthsuite to check for a heart rate. All was well, and I was reassured that babe was ok. Like the other two, this one has awake and sleeping times.
Because you're still relatively early, try giving baby a good poke, otherwise, nothing beats peace of mind, its truly priceless xx