Hi mama, 100 BPM at 6 weeks is actually on the lower side, normally baby's heart rate starts around 100-110 and goes higher as the weeks go on, peaking around 160-180 BPM mid-pregnancy. I'd really suggest checking with your OB about this kasi they can do a proper ultrasound and make sure everything's okay, especially since heart rate can be tricky to measure accurately that early. One of my mum friends had similar concerns at her 6-week scan and ang doctor lang talaga ang makapag-confirm if normal or hindi, so don't worry too much yet but best to get it checked out soon para sa peace of mind mo!