Firstborn sex defines early childhood growth of subsequent siblings

Animal studies have shown that maternal resource allocation can be
sex-biased in order to maximize reproductive success, yet this basic
concept has not been investigated in humans. In this study we explored the
relationships between maternal factors, offspring sex and prenatal and
postnatal weight gain. Sex-specific regression models not only indicated
that maternal ethnicity impacted male (n = 2456) and female (n = 1871)
childrens’ postnatal weight gain differently but also that parity and mode
of feeding influenced weight velocity of female (β ± S.E. = -0.31 ± 0.11
kg, P < 0.001; β  ± S.E. = -0.37 ± 0.11 kg, P <
0.001) but not male offspring. Collectively, our findings imply that
maternal resource allocation to consecutive offspring increases after a
male firstborn. The absence of this finding in formula fed children
suggests that this observation could be mediated by breast milk. Our
results warrant further mechanistic and epidemiological studies to
elucidate the role of breastfeeding on the programming of infant growth as
well as of metabolic and cardiovascular diseases, with potential
implications for tailoring infant formulas according to sex and birth
order.