What week of pregnancy am I in

by Desiree
(New Mexico)

What week of pregnancy am I in? I am curious to know because I am not sure when I actually conceived and my period is very irregular.

The doctor gave me a due date of October 5, 2010. He started me on some prescribed prenatal vitamins and we scheduled the first fetal ultrasound. So I am only a few weeks pregnant. How can they know for sure what my due date will be if I'm not even sure when I ovulated or conceived. I just want to get my time line straight so I'll know exactly how long I have before the baby comes.

Do they have some kind of test that can more accurately determine my progression? This is my first pregnancy so I'm just a little anxious.