CRL stands for Cephalic-Rump Length, which is the distance from the top of the fetus’ head to the buttocks as measured by ultrasound. CRL plus 6.5 equals the week of pregnancy, so CRL is often used to check the week of pregnancy, and the most accurate CRL is measured by ultrasound at 11-13 weeks of pregnancy. In mid-pregnancy, the week of pregnancy should be rechecked according to the patient’s menstrual status, early pregnancy ultrasound, and the head-butt length measured by ultrasound at 11-13 weeks of pregnancy. If the difference between the week of pregnancy according to ultrasound and the week of pregnancy according to the last menstrual period is large, and the difference is more than one week, the week of pregnancy according to ultrasound should be used as the basis for rechecking the week of pregnancy.