Why should pregnancy be counted from the first day of the last menstrual period

Pregnancy is calculated from the first day of the last menstrual period, which is a unified medical regulation, because each person, each month's ovulation time is not fixed, so it is impossible to determine which day is pregnant, and it is impossible to calculate the expected date of birth according to the day of conception. According to the first day of the next menstrual period, the expected date of delivery is about 280 days of pregnancy, which is also 40 weeks of pregnancy. If the time of pregnancy is clear, various pregnancy examinations can be done on time, such as NT examination at 12 to 13 weeks of pregnancy, Down screening at 15 to 20+6 weeks of pregnancy, and non-invasive DNA, thalassemia screening and other pregnancy examinations.