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.

Between January 2007 and February 2018, data were obtained from electronic medical records from 33874 admissions to the Pediatric ward of a metropolitan teaching hospital in Western Sydney, Australia. Entries of children that had been transferred from the obstetric ward to another ward, within a week after birth were excluded (n = 5111), as were entries of children older than five years of age at admission (n = 5611). Several children (n = 5762) had been admitted multiple times. To ensure that every child only entered the analysis once, the latest medical entry with anthropometric measures was chosen and the remaining entries for that child were excluded, leaving medical records for 16355 children.

Obstetrix^TM (59), a state-wide mandatory database collecting obstetric data, included information on the mother, pregnancy related information and infant related information such as anthropometric measurements for 67268 infants delivered at the hospital between 2000 and 2017. Obstetrix^TM was matched with the pediatric database using the children’s individual medical record number.

The included variables were screened for two-way variable interaction utilizing generalized linear models. If several interactions included the same variable the possibility of three-way interactions was investigated. Entering variables and variable interactions into a stepwise backward eliminated multiple regression model ensured that only the best sets of predictive variables were included in the final model. Variables with a P-value ≥ 0.1 were removed through backward elimination. The regression models were then followed up with groupwise comparisons, utilizing β-values for all statistically significant [SS1] variables in the regression model (except the investigated grouping variabels) to adjust postnatal weight. Grouping children by firstborn sex (if applicable), infant sex and feeding method an ANOVA was conducted. ANOVA was followed up with post-hoc Bonferroni tests to investigate the specific effects of feeding method, firstborn sex and infant sex on postnatal growth.

Yes. Data is complete. Pls refer to data code sheet.