I didn't have a clue with my first, but with my second I just knew and took a test a full week before AF was due and had a BFP. With my third it was similar in that I kinda knew that there was something going on inside, and took a test which came back +ve. The wierd thing about that one was the fact that I hadn't had a period for about 18 months, so doing an HPT seemed a little pointless, but I had a gut feeling that something was up. I had to go for a dating scan 4 weeks later because of the fact that I had no dates of LMP to go from, and it turns out that I must have POAS when I was about 2 anna half to 3 weeks pregnant! I guess I'm incredibly lucky that I have very compliant wee wee!