So confused about dates

[Milliegirl25]

My last period was August 23rd, today's October 16th. Ovulation was confirmed by hormone testing, and I tested positive at 8dpo which was 3 weeks and 2 days ago. I got my first scan today and they said I was measuring behind at 6 weeks and 1 day, rather than 7 weeks and 6 days.
How does this happen when ovulation date was confirmed? If im only 6 weeks 1 day that means I got a bfp at just under 3 weeks! Is that even possible?
I got a bfp at 8dpo so must have implanted at 6dpo which was 3 weeks and 4 days ago.
My cycles are usually 35 days.

When I got my early scan done at 6 weeks 6 days (calculated by using last period) I was measuring at 6 weeks 2 days. They told me it was too small to measure accurately and to just go with my dates. When I had my 12 week scan I was measuring just one day behind the dates I’d calculated! So I would wait until the 12 week scan to properly know about dates. 😊

I think its your long cycle.
If you take ovulation as mid cycle (of the 28 day variety the numbers all assume), you are 3+2 past positive test, +8 from ovulation, so 4+3, plus a further 2 weeks to an assumed period. That would make you just over 6 weeks.
Your cycle is a week longer than the calculations allow for, giving you the 7 weeks you'd calculated.
Sounds right to me.

If you know when you ovulated, I think your dates and calcs will be right for gestational age. The dates you’ve been given at scan are based on embryo measurement (I think), which as PP said can be inaccurate at an early stage. It might also be that growth is a little behind, but from what I’ve read this can either catch up or just carry on at a normal rate after a slower start.

Just remember they're dealing in fractions of millimeters at this stage and it's really not unusual to measure ahead or behind.
They often refer to the 12 week scan as the 'dating scan' as this is a much more accurate time for measurements

Hello fellow 35 day cycler! Having a slightly longer cycle is annoying as everything is measured based on an assumed 28 day cycle, which we know is not correct for us. I went for my 12 week scan knowing I was 11 weeks- got in and was told "you're only 11 weeks, you'll need to come back next week". I guess because so many women don't know their accurate dates they just have to go with a standard but it's annoying!

But anyway yes, sounds right to me based on your dates, congratulations!