Pregnancy is typically measured in weeks starting from the first day of your last menstrual period (LMP), even though conception occurs about two weeks later. A standard pregnancy lasts about 40 weeks, or roughly nine calendar months. You can divide the weeks into three trimesters: first (1-13 weeks), second (14-26 weeks), and third (27-40 weeks). Online pregnancy calculators or your doctor can help determine your exact week and due date.