Fetal doppler 12 weeks

Erin • Mama to Owen + baby coming June 2023 💕

I went for my 12 week appointment at doctors yesterday and they confirmed everything is good via ultrasound. Heart rate 150+. I purchased a fetal doppler that came in today and the only thing its picking up is max 135. Does anyone have tips for using a doppler at this stage? Does it show up slower than on an ultrasound?