pregnancy

I don't understand the whole when your actually pregnant or not thing and how they go by your last period. suppose you loose your virginity when your ovulating, which is two weeks after your last period, how do they use your last period of you haven't even had sex yet? so suppose I had sex 2 weeks ago and my period was about a week prior and my next period is coming up next Friday, can I take a test this Monday and it be accurate?