DOI: 10.5061/dryad.4b8gthtbg
title: Firstborn sex defines early childhood growth of subsequent siblings

description: 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 &lt; 0.001; β  ± S.E. = -0.37 ± 0.11 kg, P &lt;
 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.
description: 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<sup>TM </sup>(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<sup>TM</sup> 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.
description: Yes. Data is complete. Pls refer to data code sheet.