The earliest age at which rickets begins and the precise time required for the development of the pathologic process in the bones are not definitely known. Craniotabes, the first clinical sign of involvement of the skeleton, has been described in the second month; ordinarily, however, it does not manifest itself before the third or fourth month. By the fourth month, the enlargements at the lower ends of the radius and ulna and at the costochondral junctions may have attained such size as to be recognizable on physical examinaiton, but then the rachitic process is already advanced. Craniotabes makes its appearance at a time when the pathologic process at the ends of the ribs and long bones of the extremities is in an early stage of development, and it is the earliest positive sign of rickets.
The difficulty in regard to the value of craniotabes in the diagnosis of rickets lies
DUNHAM EC. RICKETS IN AN INFANT OF THIRTY-FOUR DAYS. Am J Dis Child. 1923;26(2):155-163. doi:10.1001/archpedi.1923.04120140054005