Mammary gland function during gradual weaning and early gestation in women

Daily changes in the composition of the milk of two lactating women were recorded from the time of the last menstrual period prior to conception throughout the first two months of gestation. The changes in the composition of the milk from these women were compared to the daily changes in the milk composition of two women during the period of gradual weaning. In the weaning subjects the concentration of sodium and total protein had increased 14- and 1.6-fold, respectively, by the end of the study. The concentrations of potassium, glucose and lactose decreased 1.3-, 149- and 3.6-fold, respectively, from before weaning commenced to the end of the study. These changes were closely correlated to the decrease in the suckling frequency of the infant. The concentration of sodium and total protein in the milk from the unsuckled breast of two women was 11- and 2.8-fold higher than in the milk from the normally suckled breast of the same woman. The concentrations of potassium, glucose and lactose were 1.1-, 70- and 1.9-fold lower, emphasizing the necessity of a regular suckling pattern on the maintenance of lactation. The concentrations of sodium and total protein similarly increased during pregnancy, being 8- and 3.8-fold higher by the second month of gestation. The concentrations of potassium, glucose and lactose decreased 1.4-, 73- and 4.4-fold, respectively, over the same period of time. The changes in the milk composition showed no significant correlation to the infant's suckling frequency. The results indicate a progressive loss of secretory activity by the mammary gland during weaning and early gestation. During weaning this loss appears to be directly related to the decrease in the infant's suckling frequency, whereas during early gestation there would appear to be an inhibition of milk secretion which over-rides the positive stimulus provided by the infant's suckling. The mechanisms regulating the secretion of milk during weaning and pregnancy are discussed in the light of the compositional changes observed.