fetal heart rate at 17 weeks

Hi ladies I'm 17 weeks today and my baby's heart rate was 135.My original 9 weeks appointment for heart rate was 176. And throughout the weeks my baby's bear rate has been 145-150 and sometimes higher. I am a first time mom so seeing anything like this freaks me out. Is this a normal fluctuation in heart rate? I heard the baby's heart gets slower with time but I'm just nervous it's getting slower and slower.