The parietal-rump length is the length between the highest part of the fetal head and neck and the lowest part of the fetal buttocks and is an important indicator for assessing fetal physical development. Prior to the 7th-12th weeks of pregnancy, fetal physical development is assessed by measuring the fetal parietal-rump diameter. If the length of the parietal-rump diameter corresponds to the actual gestational week, the fetal growth rate is normal; if the length of the parietal-rump diameter is significantly less than the normal reference value of the actual gestational week, the fetal growth rate is considered to be slow, which may be caused by insufficient intake of various nutrients during pregnancy, or by serious pregnancy complications such as hypothyroidism, severe anemia, etc. that affect the physical development of the fetus. If the exact number of weeks of pregnancy is not available, the approximate week of gestation can also be estimated by measuring the length of the fetal parietal and rump diameter.