On the basis of a study of X-ray films of the lower limbs of 994 babies (included 245 hypotrophics) a "bone age" scale of the fetus between 28 and 42 weeks of gestation has been established. The Acheson's method has been used and the bone age determined by addition of notes given to different evolutive phases of the first five ossification centers of the ankle and the knee. To more practical purpose, the results of boys and girls have been joined, although the bone age is in advance among the girls and a theorical scale established with the 10th, 50th and 90th percentiles curves. The comparison with the curves established among 245 hypotrophic newborns, seems to prove that the bone age is a more resistant criteria to malnutrition than the weight and even the length. This new criteria to determine the fetal age does not escape to the criticisms given to the other existing criteria, but radiograph is an objective and fixed document. Its repetition allows to follow the growth of the child and probably to study the role of various factors, mainly nutritional, on this growth. The measure of the length of the tibia and of its increament on the same X-ray films is an other possible method of great interest for the same purpose.