The standard value of fetal femur length during pregnancy varies at different gestational weeks. At eighteen weeks of pregnancy, the fetal femur length is around 2.4-2.7 centimeters. At 28 weeks, it is about 5.1-5.2 centimeters. At 38 weeks, it is about 7.0-7.2 centimeters. Moreover, there is a direct relationship with maternal nutritional absorption and the size of fetal development. These data can only be used as a clinical reference and cannot be used to directly determine the size of the fetus based on these values.