How far gone am I ? Early pregnancy and can't work this out

[Mell5377]

I had an early scan today which was according to my last period, at 6 weeks, however, they said the sac size looked 4-5 weeks.

I know that I ovulated late the this time I conceived so it's been about 21 days since I ovulated and conceived according to my OPK. Would that explain the sac looking small? I'm not as far along as 6 weeks then?

I'm having a rescan in 2 weeks

Length of pregnancy is calculated from the first day of your LMP. It is assumed that ovulation takes place around day 14, but in any event ovulation happens 14 days before the first day of the next period.
If you are certain you ovulated late, it is perfectly possible that you are 6 weeks by dates, but less than that on the scan.