baby not as active

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So my baby was very active at around 13-15 weeks and I just turned 16 weeks and now my baby doesn't seem to be as active and he use to be. I'm sure his fine because I check he heart beat everyday. But why don't I feel his kicks any more? I feel flutters every once in a while now not everyday like I use too. has Anyone or has experience this. I just want to know if this is normal or what.