Some work in Harlem Hospital in which we were attempting to ascertain the factor that predisposes children to rickets by a systematic survey of the phosphorus of the blood of mothers and infants suggested a study of wider scope, including not only the inorganic phosphorus but also other constituents of the blood during the critical period of child formation in the uterus. Early investigators have had continual controversies regarding the significant chemical changes that occur in the inorganic constituents of normal pregnant women. The recent work of Denis and King1 in which from 9 to 11 mg. per hundred cubic centimeters of blood is accepted as representing the standard for normal serum calcium is corroborative of these earlier studies in which no change was predicated. Of all the workers in the field, however, Widdows2 is the only one who reaches her conclusion by analyses of the blood of the same