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 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 = 0.005; 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 formulae according to sex and birth order.

中文翻译：

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长子性别决定了后续兄弟姐妹的儿童早期成长

动物研究表明，母体资源分配可能存在性别偏见，以最大限度地提高繁殖成功率，但这一基本概念尚未在人类身上进行过研究。在这项研究中，我们探讨了母体因素、后代性别与产前和产后体重增加之间的关系。性别特异性回归模型不仅表明母亲种族对男性（n = 2456）和女性（n = 1871）儿童产后体重增加的影响不同，而且胎次和喂养方式影响女性的体重速度（β±se = -0.31 ± 0.11 kg，p = 0.005；β ± se = -0.37 ± 0.11 kg，p < 0.001）但不是雄性后代。总的来说，我们的研究结果表明，在男性长子之后，母亲对连续后代的资源分配会增加。在配方奶喂养的儿童中没有这一发现表明这一观察结果可能是由母乳介导的。我们的结果需要进一步的机制和流行病学研究，以阐明母乳喂养对婴儿生长发育以及代谢和心血管疾病的作用，并可能对根据性别和出生顺序定制婴儿配方奶粉产生影响。