1)If the menstrual cycle is around 30 days, the pregnancy week is counted from the first day of the last menstrual period, the total number of days divided by 7, the whole number is the number of weeks, the remaining number is zero, such as the last menstrual period 2015-5-10, 143 days of menopause at 2015-9-30, recorded 20 weeks of pregnancy + 3; 2)Irregular menstruation, or the last menstrual period is not remembered, only through ultrasound examination to measure the size of the gestational sac or embryo head and hip length The maximum diameter of the gestational sac (cm) + 3.5 = gestational week, the head-rump diameter (cm) + 6.5 = gestational week, the biparietal diameter X 4 = gestational week; 3) Modern ultrasound machines have the function to report how many gestational weeks directly after measuring each diameter, so the gestational weeks can be projected based on the results of multiple examinations.