No heartbeat at 9 weeks

Hi I just saw gynae for the first time at 9 weeks. Gynae couldn’t hear a heartbeat and saw some implantation bleeding.. advised to come back aft a week to double confirm.. if it’s a miscarriage. Anyone experienced this bfr? Any chance it’s a scare and it’s too early to tell? That I could have calculated my weeks wrongly? Gynae said it may be too early to tell as I don’t Remb what exactly is too small.. 4mm.