Calculating due date??

If they go by the first date of last period when determining gestational age, what happens in the rare occasions when women conceive on their period? Or another time other than ovulation? Would that change the due date, because wouldn’t the baby be smaller/bigger?

It’s got me all confused.

I went into my appointment today and baby was measuring exactly 8 weeks, which is spot on with my period being November 24th and conception December 10th. But what if I was to conceive like, December 1st and the baby was bigger. Would that change the due date and gestational age?