medwireNews: Researchers attribute the increased growth in boys relative to girls in early infancy to the testosterone surge that occurs during “mini-puberty”.
In the first part of their two-part study, Leo Dunkel (Queen Mary University of London, UK) and colleagues compared the growth trajectories of 9468 boys and 9102 girls over the first 12 months of life.
Confirming previous studies, they found that growth velocity decreased in both genders over this time, but with significant differences over the first 6 months, in that boys grew faster than girls. The largest difference was observed at 1 month, when boys grew an average of 4.1 cm/year faster than girls.
This difference between the genders “is of the same magnitude as in puberty”, the researchers comment in Pediatrics.
The second part of their study involved 84 infants (38 boys; 46 girls), who had serial urinary testosterone measurements. Testosterone levels rose in both genders between 7 days and 1 month (ie, the start of mini-puberty), but were significantly higher in boys than girls through to the age of 6 months.
Testosterone levels correlated significantly with growth velocity in the whole cohort over the first 5 months. The correlation was strongest at 2 months, at which point testosterone level accounted for 24% of the variance in growth velocity in boys and 48% in girls.
The association between testosterone and growth velocity persisted in a statistical model that also included oestradiol and insulin-like growth factor (IGF)-1 levels, gestational age at birth and birth weight and length standard deviation score. The strongest association was, again, at 2 months.
IGF-1 levels were associated with growth velocity at 3 months only, and oestradiol was not associated with growth rate at any time over the first 6 months of life.
In a related commentary, Kenneth Copeland and Steven Chernausek, from University of Oklahoma Health Sciences Center in Oklahoma City, USA, call the research “convincing” and say that the magnitude of the effect of mini-puberty on growth “is not trivial.”
They say the between-gender difference at this age “reasonably explains the 1.9-cm mean difference in lengths between boys and girls at 12 months”, and also accounts for about 15% of the height differences between adult men and women.
By Eleanor McDermid
Pediatrics 2016; 138: e20153561
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