is it normal to not see a heartbet on my 6th weeks?

I should be 7week and 6days if base on my lmp but base on my utz im only 6 weeks and the embryo is there but no heartbeat. Im starting to worry. last week I had my 1st checkuo and there is a sac but no embryo. yesterday was my follow up checkup and it has an embryo but no heartbeat. can someone explain if its normal or if I need to prepare myself for miscarriage