All regulations1 standardizing cow's milk for human consumption exclude its use for a period of time before parturition, the period varying from fifteen to sixty days. Standards of this character are based on the supposition that pregnancy influences the normal composition of milk; in other words, that the milk no longer has a normal composition after gestation has reached a certain stage. The evidence on which this supposition is based is, however, extremely difficult to find, although the literature is extensive with regard to variations in the composition of milk.
The question of the factors influencing the normal composition of cow's milk has for several years been the object of critical study by the Missouri Agricultural Experiment Station. Certain portions of data, which were obtained for another purpose,2 give a clearer insight into the importance of gestation as a factor causing milk of abnormal composition than any data
PALMER LS, ECKLES CH. GESTATION VERSUS STAGE OF LACTATION AS FACTORS CAUSING ABNORMAL COMPOSITION OF COW'S MILK. Am J Dis Child. 1917;13(5):413–419. doi:10.1001/archpedi.1917.01910050019003