[Closed] Obstetrics Q - any experts help with this please?

Mrs abuhamster has just had her 12 week scan. I was expecting this to confirm that the foetus is 10 to 12 weeks old. However, it has been measured and the age of the foetus has been determined at 14 weeks and 6 days. They are saying she is 15 weeks pregnant.

Now, the problem is that I was away 14 weeks and 6 days ago. If the foetus is older than 12 weeks and 6 days then it isn't mine!

You can probably imagine the conversations we have had. Can anybody please advise how accurate this ultrasound measurement is please?

Thanks for any advice!