Hi am still late on my period for 7 days this month,and I took 3 pregnancy test confirming I'm pregnant. How many weeks pregnancy that can be?

Figuring when conception occurred is only an estimate (unless you only had sex once; even then it can be up to 3 days later). Your doctor will estimate based on the possible middle of the month following your last period that ovulation often happens, but not always. So due dates can be off by a few weeks, and you just can't pinpoint conception.