What is considered a low fetal heart rate

Patient: I had a first ultrasound today at 8 weeks 5 days pregnant. Found out I am having twins. Baby A has a heart rate of 180 but baby B only has a heart rate of 124. My dr said that is a little concerning as it is low for its gestational age. My question is weather a heart rate if 124 is considered low for a twin at 8 weeks that is measuring at 8 weeks 3 days . Thank you.