The fetal parietal-rump length is the length between the top of the fetal skull and the fetal rump measured by ultrasound between 8 and 12 weeks of pregnancy. The parietal-rump length is an important indicator for ultrasound, which can estimate the size of the fetus and generally determine whether the fetus is developing normally. It can also be used to check the date of pregnancy, especially for women who cannot remember their last menstrual period, have irregular menstruation or are pregnant without menstruation during breastfeeding, the most accurate indicator to estimate the gestational week is to measure the fetal top-rump diameter through an ultrasound in early pregnancy.