Determination of Crown‐Rump length from fetal long bones: Humerus and femur

Abstract Lengths of the ossified diaphysis of the humerus and femur were measured in 50 fetuses ranging from 65 to 290 mm Crown‐Rump length. A significant correlation was found between the diaphyseal length and the CR length. No appreciable difference was noted in diaphyseal lengths of the humerus and femur in fetuses of 85 mm CR length or below. Various growth phases were observed. The difference in length of the femur and humerus is due to a comparative slow growth of the humerus during the later period of prenatal life. Diaphyseal growth rates of the humerus and femur are 0.18 mm and 0.21 mm respectively for every 1 mm increase in CR length. Regression coefficients for the lengths of the ossified shaft of the humerus and femur are 5.35 and 5.00 respectively. With the help of these coefficients, CR length of the fetus was estimated within a range of ± 15 mm. From the estimated CR length, age of the fetus was determined with the help of a standard age and size curve with reasonable accuracy.