Changes in maternal breast size have also been shown to be related to quality of milk ... how girls steal some of their mother's beauty. It has to be remembered ...
Background Previous research has found an effect of infant sex on the energy content of breast milk, with 25% greater energy content in breast milk for males than for females. Changes in maternal breast size have also been shown to be related to quality of milk produced, however in one study so far, this does not appear to have been associated with infant sex (1). Here, it is hypothesized that changes in breast size during pregnancy may be related to the sex of the offspring. Males tend to grow faster in the womb (2), and tend to be heavier at birth than females (3). Therefore, due to higher nutritional demands, greater changes in breast size should occur in women expecting sons. To test this hypothesis, this study evaluated changes in breast size before and directly after pregnancy depending on the infant’s sex.
Methods Data for this study was collected using an online questionnaire targeting Polish women who had at least one child. 120 complete surveys were included in the analysis. Mothers answered questions about their reproductive history, their most recent child and details about the course of their last pregnancy. They also provided information about their body size measurements, including bra size (cup size and chest circumference) before pregnancy and directly after giving birth. Self-reported measurements were then converted into a scaled measure ‘breast circumference’.
Results Changes in breast circumference during pregnancy, estimated on the basis of self-reported bra size before and directly after the last pregnancy, were associated with the sex of the infant. However, in direct contrast to the hypothesis, mothers of female infants had greater breast circumference changes than the mothers of male infants (p = 0.03). Furthermore, in analysis of covariance, it was shown that after controlling for maternal weight change during pregnancy the observed differences in the changes of breast size depending on infant’s gender remained statistically significant (p = 0.03) [Fig. 1]. 11
Change in breast circumference (cm)
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Unexpected changes in maternal breast size during pregnancy in relation to infant’s gender Andrzej Galbarczyk
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Figure. 1. Change in breast circumference according to infant sex (controlling for maternal weight change)
Conclusion The observed difference seems surprising in the light of studies reporting that mothers of male infants produced milk that had greater energy content. Larger increase in breast size questions the old wives’ tales describing how girls steal some of their mother’s beauty. It has to be remembered, nevertheless, that breast size alone does not necessarily determine the quality and quantity of produced milk. It is likely that human females developed two strategies for milk production, depending on the infant’s sex, and possibly similar to rhesus macaque females – by synthesizing richer milk for sons but more milk for daughters (4). 1. Powe CE, Knott CD, Conklin-Brittain N. (2010) Infant sex predicts breast milk energy content., Am J Hum Biol. 22(1):50-4. 2. Loos RJ et al., (2001) Length of gestation and birthweight in dizygotic twins Lancet. 358(9281):560-1. 3. Marsál K et al., (1996) Intrauterine growth curves based on ultrasonically estimated foetal weights, Acta Paediatr. 85(7):843-8. 4. Hinde K. (2009) Richer milk for sons but more milk for daughters: Sexbiased investment during lactation varies with maternal life history in rhesus macaques, Am J Hum Biol. 21(4):512-9.
