Other risk factors and infant birthweight
Table 2 and Table 3 presents the three quantile regression models for
SGA and LGA infants, and Figure S3 presents model estimates for
continuous covariates. Infants born to older mothers were more likely to
be SGA (10th percentile: β=-15.2, 95%CI=[-23.0,
-7.34], p=0.001), but not LGA (p=0.59) and had a reduced median
birthweight (50th percentile: β=-7.5, 95%CI=[-14.9,
-0.05], p=0.05). While older fathers had a reduced risk of fathering
an SGA infant (10th percentile: β=9.5, 95%CI=[4.15,
14.8], p=0.0005). Higher SEIFA scores were associated with increased
median infant birthweight (β=44.3, 95%CI=[16.7, 71.8], p=0.002) and
a reduced risk of an SGA infant (β=32.0, 95%CI=[7.92, 56.1],
p=0.009.
Male infants had higher birthweight compared with females by a similar
amount in all regression analysis (all p≤0.03). Birthweight increased
non-linearly with gestational age with smaller increases in weight for
gestational ages >40 weeks. This tapering in the increase
in birth weight was more extreme for SGA (10thpercentile: p=0.003 and 5th percentile: p=0.004) than
for LGA (90th percentile: p=0.16 and
95th percentile: p=0.03). Infants delivered by
caesarean section were more likely to be heavier (90thpercentile: β=133, 95%CI=[62.3, 203], p<0.0002), however
their risk for SGA was not different to babies delivered vaginally
(p=0.82). There was no detectable influence of insemination method on
infant birthweight.