Fetal Heart Rate 13 weeks

So the only time I have seen my obgyn was at 8 weeks exactly the heart rate was 177. I use my Doppler pretty regularly and from about 10-11 weeks it's was consitently in the 170's to 180's. I know it is supposed to lower with time but I'm only 13+1 now and no matter what time of day i use it, whether I have just eaten or just woke up it's only about 145 sometimes hitting 150 but typically 145-149. It seems like it slowed down so much and I'm pretty worried. Is this normal?