Heart rate 100 bmp at 6w1d

Pp

My first U/S is on 6w1d with HR 100 bpm.

But normal fetal heart rate usually ranges from 120 to 160 bmp around 6w. My baby’s HR is lower than normal. doctor didn’t worry about it, he said fetal is still tiny (3.5 mm). HR will be higher in next time. But I still worry.

Is there anybody who is in the same situation as me?