Calculations?

So this might be a silly question.

Is how far along you are/your due date determined by your cycle length the month you conceived? Or is it based on average (multiple months)? I know that the start of your last period is also important.

I’m only asking because the month I conceived I had a 24 day cycle. The month prior to that, it was a few days longer. So is how far along I am, based on my cycle length that month as well as my period?

I did the sneak peak test (clinical location one). You have to be 8 weeks. I got my blood drawn at 8w3d. This is based off me having a 24 day cycle (from the month I conceived) and my last period. If it’s based off average months, I would have been 7w6d when I got my blood drawn and I’m worried that the results could be inaccurate (because I wasn’t 8 weeks).

Anyone have any insight?