Hartmann PE, Rattigan S. Saint L, Supriyana O. Variation in the yield and composition of human milk. Discussion of paper by Hartmann et al. The two women you showed us both produced more milk from the right breast than the left. Dr. Hartmann I think that perhaps two-thirds of women are uneven in the milk production between their left and right breasts, but it doesn't necessarily mean they are consistently higher in the right breast. We found that some women changed from one lactation to the next, so most milk production in one lactation is in the right breast, and in the next it's in the left breast, so there is a fairly random distribution between breasts.
Some people think it has to do with the handedness of mothers.
We haven't looked at that, but certainly it can change during subsequent lactations. It seems to me to relate to a behavioural characteristic in animals, which is teat order. There is evidence from Sweden, I believe, that indicates that when babies are put on their mother's abdomen, they make their way up to the breast. They also tend to associate with the breast that they attach to first. This observation would be consistent with observations of the establishment of teat order in other mammals.