The last menstrual period is defined as the 1st day of the last menstrual period, when vaginal bleeding first started. For women who are pregnant, the last menstrual period is a very important point in time, and the week of pregnancy is usually calculated from the last menstrual period. A normal pregnancy lasts 40 weeks from the first day of the last menstrual period, which is two weeks longer than the true embryonic week. Pregnancy is divided into 3 stages, 0-13 weeks + 6 days for early pregnancy or early gestation; 14 weeks – 27 weeks + 6 days for mid-gestation or mid-pregnancy; after 28 weeks of pregnancy until birth for late pregnancy or late gestation. Obstetric examination time is closely related to the pregnancy week, every four weeks before 28 weeks of pregnancy, every two weeks between 28-36 weeks of pregnancy, and once a week after 36 weeks of pregnancy. Obstetric examination programs are also closely related to the week of pregnancy. In China, it is routine to conduct four-dimensional screening of fetal structure between 20 and 26 weeks, and three-dimensional screening of fetal structure between 30 and 32 weeks. For women who cannot remember the date of their last menstrual period, early ultrasound can be used to verify the week of pregnancy.