Actually a bit strange too, if we count the weeks of gestation from last menstrual period. But because most women almost can not be sure exactly when they started to contain (and that's fair), so it's better doctors calculate from your last menstrual period even though you are not pregnant at least until 2 weeks later. For example, if your physician says that you are pregnant 12 weeks (based on your last menstrual period), then you begin to contain 10 weeks ago.
This is a very early weeks, if you are planning a pregnancy, you should start by thinking that you are pregnant. This means eating healthy foods, take vitamins for pregnancy that contain folic acid quit drinking and drugs, smoking, alcohol and other unhealthy foods.