How to calculate how many weeks pregnant you are If you want to know how many weeks pregnant you are, you can count from your last menstrual period. Usually, pregnancy is 40 weeks, which is 280 days, starting from the first day of the last menstrual period. If you are not sure about your menstrual cycle, you can do an ultrasound to determine how many weeks you are pregnant. Pregnancy Precautions During pregnancy, women need to pay attention to their own health and the health of their fetus. first