The femur length of 40mm at 23 weeks is not considered short. The reference value for femur length at 23 weeks of pregnancy is 40mm, and there is an error in ultrasound measurement of femur length during pregnancy, generally there can be an error of two weeks up and down, only not exceeding this range, so there is no need to worry too much. In addition to developmental abnormalities, genetic factors, insufficient calcium and high quality protein supplementation can also affect fetal femur length. When the ultrasound during pregnancy suggests that the length of the femur is low or long, in addition to ensuring that adequate calcium and high-quality protein supplementation during pregnancy, you should consult with your obstetrician in time to rule out abnormalities in the development of the fetus, such as short femur length due to genetic factors, it is recommended that after the birth of the fetus, increase calcium supplementation, balanced nutrition, and strengthen physical exercise.