Baby's heart rate at 6 weeks

Becky

When I had my first ultrasound to determine due date etc they said the heart rate was 100. I remember my prior 2 pregnancies the heart rate being much higher like maybe an average of around 140? The nurse practitioner said they like to see a heart rate of at least 100 so she said that was ok. I'm just worrying bc I'm much older with this pregnancy (40). Does this seem ok and could it have been bc it was so early in the pregnancy? TIA 😊