Purpose: Milk output from the right and left breasts was compared in mothers who were pumping exclusively and had not yet fed their infants at breast.
Methods: Thirty-five mothers of very low birthweight infants established lactation with a hospital grade, electric, dual pump, and recorded milk output separately for each breast during every pumping session from enrollment until completion of the study (mean = 19.8 days) using a standardized milk log. Milk output from each breast was also weighed during six observed milk expressions over a 2-week period during the study.
Results: For the observed pumping sessions (n = 210), milk output was greater from the right breast in 65.7% of the sessions. For the milk log data (n = 3099 pumping sessions) milk output was greater from the right breast in 47.6% of the sessions, greater from the left breast in 28.0%, and equal from both breasts in 24.4% of the sessions. The mean difference in milk output between the right and left breasts was 6.6 mL (SD = 12.1) for the observed sessions, and 5.0 mL for the milk log data (SD = 10.9). The mean right-to-left breastmilk output ratio was 1.20 for the observed sessions and 1.17 for the milk log data. The right-to-left breastmilk ratios were not associated with time of day, day of pumping, total milk output, maternal handedness or the breast pump suction pattern. The right-to-left breast differences were associated with parity and breastfeeding experience, with primiparous women and first-time breastfeeders demonstrating the greatest differences.
Conclusions: These findings suggest that differences in the milk output from the right and left breasts are common, and that milk output is often greater from the right breast. The differences appear early in lactation, are not related to total milk output, and are relatively consistent throughout the day and over the first weeks of lactation.