Weeks pregnancy confusion

[Tango500]

So I've just found out I'm pregnant ( took a test after missed period) but I'm so confused.. according to all the due date calendars, I'm already four weeks! Surely I only conceived a couple of weeks ago?

The calculators make you a couple of weeks pregnant before you've conceived as they count from the 1st day of your last period

I noticed that.. but I'm wondering why? I don't understand lol

A pregnancy is 40 weeks from the start of your last period. Average ovulation is day 14 hence as a general rule you add two weeks on and then this is often tweaked by a few days at the dating scan.

Ahh right! Now it's making more sense. Thank you