One Deadline To Week To See Heartbeat Or Else...

Ch
Hello mamas and mamas-to-be! My last period was 11 weeks ago, but I have always had irregular cycles. When I went to the OB-GYN today, he put me at 5 weeks, and made an appointment for 9 days from now since we're leaving the country for a few weeks. 
He said if we don't see a heartbeat at that appointment (via vaginal ultrasound), it is inevitable the pregnancy will end in miscarriage. He also said he saw a second, smaller sac, which was likely a second implantation that did not survive.
On these boards, I've seen many moms that said they didn't see a heartbeat until 7, 10, or even 17 weeks! So why did Doc give us this deadline? :(