Heart rate

I have my first ultrasound just today due to ECQ, the heart rate is 90bpm,.i got nervous because on what I have read, it's not normal for 9 weeks,.but my OB told me to relax and need not to be stressed,.it's not abnormal according to her, there are other pregnant women who experienced it and as the weeks go by, the heart rate of the baby becomes normal, is anyone of you experienced this?