When a doctor dates a pregnancy they date it from the first day of your last period. You will considered one month, or roughly 4 weeks, when you miss your period.
Oh, and trimesters are figured out a little differently from doctor to doctor and website to website, but usually the first three months is the first trimester, the second three is the second trimester and the last three months of pregnancy is the third trimester.
check out www.babycenter.ca - you can have weekly updates and it will give you some great info.