Confused about Pregnancy Dates on Ultrasound

Patient: Hi i dont know if you can help. i am pregnant and my last period was end of november witch would put me at around 6 weeks i went for a scan on 14th jan and they could only see the pregnancy sack and yoke but not the baby the lady said im probably 4 to 5 weeks should i be worried

Doctor: I understand how worried you are about your recent pregnancy follow up. Certainly if your last menstrual period was at t he end of November, you will need 2 more weeks after that date approximately to ovulate; therefore you should have been able to conceive around mid December.Let me reassure you that, if we follow this rule, then according to your last menstrual period you should be 4 +- 1 week pregnant. I hope I cleared your doubt.