Unexpected changes in maternal breast size during pregnancy in relation to infant sex: an evolutionary interpretation

This study evaluated changes in breast size during pregnancy depending on infant sex. Due to larger nutritional needs during lactation, male infants may stimulate greater changes in maternal breast size than female infants.

Data were collected by an online questionnaire survey of 120 women from Poland, who had at least one child.

Changes in breast circumference during pregnancy were associated with the infant's gender. Surprisingly, mothers of female infants had greater breast circumference changes than the mothers of male infants (P = 0.03).

The observed difference is surprising in the light of studies reporting that mothers of male infants produced milk that had greater energy content. However, breast size alone does not determine the quality and quantity of produced milk. It is suggested that the larger difference in breast size for mothers of female infants results from a trade-off between direct energy allocations to the growing fetus and to breast enlargement. Perhaps, as in other primates maintaining sexually attractive attributes during pregnancy, early breast enlargement in women is a primary sexual stimulus. The ultimate function of early breast enlargement may function to ensure greater involvement from partners, which is especially important to mothers expecting daughters, who might be in poorer nutritional condition, as predicted by Trivers-Willard hypothesis.