12 week scan - only 9 weeks pregnant

Gillian
Hi everyone
I am looking for some advice. This is my first baby and I went for my 12 week scan this week only to be told I am 9 weeks. I need to go back in 3 weeks. I have worked out my dates and was keeping quite an accurate diary on glow because I have PCOS and I know those dates don't match. Is it likely they could at least be 1 week out and I'm actually 10 weeks now?
Any help would be great.
Thanks and happy weekends everyone!