I'm 36 Years Old And Should Be 9 Weeks 1

I'm 36 years old and should be 9 weeks 1 day pregnant today. Last week at 8wks 4days ultrasound showed a sac and fetal pole they dated at 6weeks 1 day. Is there any way to know the likelihood of my circumstance being a missed ab? I have another ultrasound scheduled for the end of this week, but just curious as to how often this is an actual problem with the pregnancy or how often just an issue with the ultrasound dating?