Dating scan


Hi all OK so I had my first scan yesterday and I thought I was 12 weeks and 1 day but they said 13 weeks and 1 day

My first day of my last af which was very heavy so deffo af was 16th November which was 12 weeks ago

( 16.11.15 - 9.2.16)

I was <a href="">tracking ovulation</a> and picked it up on cd 13 which was 28th November we BD on 27th and 28th that month and only the 1st and 4th of November and on october 28th snd 30th on previous cycle

I'm confused to how they make it 13 weeks I'm not complaining and no question about the dad so it really don't matter just wondering x