Background Recent studies on the field of evolutive biology have shown that the individual variability in breast milk (BM) composition may reflect mother or neonate characteristics. These results are focused on Trivers and Willard’s evolutive theory (1973): natural choice may favour the inversion of the breastfeeding pattern.

Objective To prove that BM composition may be altered on the basis of neonate sex, mother age or number of children.

Study Design/methods Descriptive study of BM composition donated in a hospital breast milk bank from April 2013 to January 2014. Different variables were registered: Nutritional, mother (age) and neonate variables (sex and order of birth).

Results 280 BM donation sets performed with 79 mothers were analysed. From the overall of women, 46,8% were mothers who had daughters and 53,2% had sons. It was their first child in 37 mothers. It could not be observed the correlation between mother age and nutritional values. In respect to the order of birth, it has been shown that the amount of fatty acids is much higher in first borns than in each child born subsequently (p0,027). Nevertheless, it has been shown that BM obtained from mothers who had daughters has a much higher protein (p0,009), lactose content (p0,055) and caloric value (p0,05) than the one obtained from sons.

Conclusions BM composition is affected by sex and order of birth. The fatty acids concentration diminishes with the birth of each subsequent child. Therefore this research has shown nutritional differences in BM composition, favouring daughters.