Consistent Circadian Variations in Creamatocrit over the First 7 Weeks of Lactation: A Longitudinal Study

Objective: The objective of this study was to test the hypothesis that fat content of expressed human milk from mothers of preterm infants is higher in samples expressed in the evening than in the morning during the first 7 weeks of lactation. Methods: The authors collected samples of expressed human milk obtained from 22 mothers of growing preterm infants, born at 26 to 31 weeks gestation, who routinely expressed all their milk every 3 hours using breast pump from the beginning of the second week to the seventh week after delivery. One aliquot was obtained from the first morning expression and the second from the evening expression. The entire aliquot was collected and mixed, and creamatocrit (CMT) was measured in a capillary. Results are expressed as mean +/- standard deviation (SD), and analyses were by repeated measures analysis of variance. Results: Mean CMT was significantly higher in evening than morning samples during the whole lactation period, week after week (p < 0.0001). Neither CMT values nor the morning-evening difference in CMT values correlated with gestational age, birth weight, or week of lactation. Morning CMT correlated significantly with evening CMT (R-2 = 0.28, p < 0.0001). Conclusions: Circadian variations in CMT are consistent during the first 7 weeks of lactation. The authors speculate that if higher caloric content expressed human milk is needed in a specific preterm infant, evening samples should be used preferentially, if available.