Confused about pregnancy?

I am so confused. I'm pregnant and I don't understand the medical concept of what week I'm in. I have my first obgyn appointment on Wednesday the 21st, but I want to fully understand what's going on before I go.

Ok, so the first day of my last period was Sept 16, 2009. I had sex with my husband the night/morning of Sept 30th/October 1st. So I must have been ovulating and concieved then. I was due for my next period on October 14th and obviously it never came and I took a home pregnancy test (actually 3) and they were positive. So in my mind I would be 2 weeks, 4 days pregnant. Is that correct?

However, on babyzone.com and a few other websites, it seems as though they are calculating my pregnancy week by the first day of my last period (sept 16) saying I'm in my 5th week of pregnancy. Which one is right? And if babyzone is right, how can I be in my 5th week when I was on my period 5 weeks ago? Is my baby 5 weeks old? I'm so confused, please help. Thanks!

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