Breast size and asymmetry during pregnancy in dependence of a fetus's sex

Objectives Breast size and fluctuating asymmetry (FA) are related to women's biological condition, as size correlates positively with fecundity, whereas FA correlates negatively with biological quality. We tested if breast volume, FA, and their changes during pregnancy are related to a fetus's sex. Women with bigger, symmetrical breasts, with a greater increase in size during pregnancy, should be more likely to carry a more ecologically sensitive and energetically demanding male fetus. Methods Ninety‐three women participated in a 3‐stage longitudinal study. 3D breast scans were performed in the first, second, and third trimester of pregnancy. As there was a small variation in pregnancy week at each research stage between the participants, the expected breast volume and FA values for the 12th, 22nd, and 32nd pregnancy week were calculated, basing on the obtained measurements. Those values were compared between mothers who carried a boy and mothers who carried a girl. Results Although women who carried a boy had somewhat larger breasts at each trimester than women who carried a girl, the difference was not significant. ANOVA for repeated measurements revealed a greater breast size increase in women carrying a boy ( P  = 0.039). FA decreased during pregnancy, but was not related to a fetus's sex. Conclusion Pregnancy‐induced breast volume increase is a better cue of a fetus's sex than breast asymmetry or breast size per se, i.e., the traits that are supposed to indicate a woman's biological condition. Women with a larger increase in breast size during pregnancy are more likely to carry to term a more ecologically vulnerable male fetus. Am. J. Hum. Biol. 27:690–696, 2015. © 2015 Wiley Periodicals, Inc.