The week of pregnancy can be calculated from the first day of the last menstrual period, or it can be measured by ultrasound to estimate the week of pregnancy. Weeks of pregnancy can be calculated from the first day of the last menstrual period to the current pregnancy, the total number of days divided by 7 to get the number of weeks of pregnancy, which can be written as “x weeks x days”, through this method can be roughly deduced the expected date of delivery for the first day of the last menstrual period, the date of the month plus 9 or minus 3, the date of the date plus seven days. However, the week of pregnancy and the expected date of delivery deduced from the last menstrual period may not be accurate, and the week of pregnancy can be corrected by ultrasound. In particular, the measurement of fetal parietal and breech diameters at 11-13+6 weeks of gestation is the most accurate method of determining the week of gestation, which is equal to the parietal and breech diameters (cm) + 6.5. When the pregnancy is ≥14 weeks, it is necessary to use biparietal diameters, head circumference, abdominal circumference, and femur length to comprehensively determine the week of gestation.