fetal heart rate at 16 weeks

so, today I had a nurse friend use the fetal Doppler because I'm terribly impatient and didn't want to wait another week for my next appointment. it took her a while to find the heart rate, and when she finally did it was only measuring at 120 bpm. she admitted that that was pretty low but said that baby could have just been sleeping. have any of you experienced such a low rate? if so, did everything turn out ok? I can't help but be a little worried since my sons was never that low