Pregnancy is kept track of in trimesters, so it's easier to monitor and check the development for you and the baby. First trimester is weeks 1 through 12, second is weeks 13 to 26, and the final is week 27 to 40, or whenever the baby's due.
And I feel obligated to say that every pregnancy's different and all I have to go by is some experience and what I've read in books, you're probably better off asking your doctor about all of this.