The correct term should be fetal vertex-rump length, which refers to the length between the top of the skull and the buttocks of the fetus measured by ultrasound during pregnancy, and this value can estimate the size of the fetus, roughly determine the development of the fetus as well as estimate the week of pregnancy. In general, there are some normal reference values for fetal vertex-rump length, which is 6-7 cm at the end of 12 weeks, 12 cm at the end of 16 weeks, 16 cm at the end of 20 weeks, 21 cm at the end of 24 weeks, 25 cm at the end of 28 weeks, 28 cm at the end of 32 weeks, 32 cm at the end of 36 weeks, and 36 cm at the end of 40 weeks. Fetal parietal length has a certain clinical significance, for pregnant women with delayed or irregular menstruation, the measurement of parietal length in early pregnancy can be used to estimate the actual week of pregnancy, which is very important for the obstetrical examination during pregnancy and the determination of the expected date of delivery.