Need some positive vibes

Soo I went to for Ultrasound yesterday to my Doctor, as per lmp my pregnancy is 8 week 2 days, but yesterday in Transvaginal Ultrasound, Dr Said only Yolk Sac and Fetal pole is showing with no Cardiac activity, also i am measuring 2 weeks behind in Ultrasound, i have irregular periods also. Doctor asked for followup Ultrasound next week.. Can anyone help to confirm the maximum time for fetal heartbeat? I have one miscarriage history I will be first time mom..