The length of the femur is measured according to the different gestational weeks, and is determined with the corresponding normal reference range of the gestational weeks, and less than 5% of the femur length in the same gestational week is considered as short femur. This is mostly done in the middle of pregnancy during a major ultrasound examination, where a preliminary determination can be made, combined with prenatal screening or prenatal diagnosis, to find the cause. At the same time, some genetic factors should be considered, because some short femur may be related to the genetic factors of parents, and may also be related to the abnormal development of fetal bones, such as osteogenesis imperfecta or chondrodysplasia, which may lead to short limbs, and requires comprehensive understanding and judgment, dynamic observation, before and after comparison and analysis, combined with whether the bones are deformed, and whether other deformities exist, to make a diagnosis and determination. The diagnosis should be made in conjunction with the presence of other deformities.