Cephalic-rump length is the length from the top of the fetal skull to the outer edge of the fetal buttocks, usually measured around 7-12 weeks of pregnancy. 1. Measurement standard: the longest and straightest midsagittal section of the trunk is taken for measurement, and the distance from the outer edge of the top of the fetal skull to the outer edge of the skin of the buttocks is measured, excluding the fetal limbs and yolk sacs, and the average of three measurements is taken in general. 2. In early pregnancy, the head-buttock length is usually used to estimate the gestational age of the fetus, which is calculated by adding 6.5 to the head-buttock length of the fetus to obtain the gestational week. If the color Doppler ultrasound reveals a head-rump length of 5.5 cm, then the gestational week is 12 weeks. By using this method, the exact date of the last menstrual period can be calculated. 3. If the number of days of gestation derived from the last menstrual period does not match the number of days of gestation derived from the head and hip length, the number of days of gestation derived from the head and hip length will be taken as the number of days of gestation derived from the head and hip length.