How accurate are ultrasounds in first trimester

I had a early scan at 6 weeks 4 days which gave me due date of 14th June. On October 24th

& ultrasound on December 10th

At 13 +4 and due date was changed to 12th June.

I know theres not much difference in the dates for EDD.

but my period started on 1st September

But calculating online brings my period to 6th September.

Also have pcos and no period for 5/6 months up until September but my anxiety is taking over. Everything ive read gives different & in my mind i feel like its all been dated wrong.

Have i pretty much ovulated later as cycles are sooo irregular & this baby brain has officially made me lose ny mind🤦🏽‍♀️😂