confused about how pregnancy weeks are calculated

Noleen • Married summer 2015
Hi, without sounding stupid how do you know how many weeks you are? I know they add two weeks on from date of conception, however my cycles were shorter, 23 days and I ovulated on CD 9 with an LH of 14 days. I ovulated exactly 4 weeks ago today and I know most people would add 14 days, do I only add 9? I tested positive on CD20, 10DPO and got the below last Thursday which would have been 3 weeks and 2 days past ovulation but only 4 weeks and 4 days from my last period. 
I'm so confused lol