What CakeBeret said. And even if you count those two weeks as part of the pregnancy, it really works out to about 9 months 1 week, so it's still pretty much 9 months. They date it as they do because using the LMP, which is usually about 2 weeks before conception, is more accurate than most other methods they can do easily to pinpoint the baby's due date (early ultrasounds work fairly well, but not every woman knows she's pregnant early enough to get an early ultrasound, and the later they are the less accurate they are for dating purposes).