Here's a brief description on the three trimesters of pregnancy. This is an exciting time in your life... If this is your first foray into pregnancy, congratulations!
Pregnancy is divided into trimesters. The average pregnancy lasts anywhere from 38 to 42 weeks, with an average of 40 weeks. Your due date will be set 40 weeks from the first day of your last period.
The important thing to remember (and that most of the medical establishment seem to forget) is that the due date is just an estimate. A baby is considered on time if it arrives ANY TIME between 38 and 42 weeks. It is considered safe to have a baby birth at home as early as 37 weeks, but in general, the baby needs every day inside that it can get. Babies born before 36 weeks are considered premature. Post-mature babies are rare, even in pregnancies that stretch the 42 week date. There are some women that carry routinely to 43 or even 44 weeks, although with today's medical climate, they are not allowed past 42 weeks unless they fall through the cracks.
The first 3 months stretches from the first day of your menstrual period in the cycle you conceive in to the 14th week. The second goes from week 15 to week 28, and the third lasts from week 29 to week 40+.