Introduction The aim of this study was to explore the age at which socioeconomic inequalities in child height emerge among children from a middle-income country.

Methods Using data from the 2004 Pelotas cohort study from Brazil we modelled individual height growth trajectories in 2106 boys and 1947 girls from birth to 48 months using a linear spline mixed effects model. We examined the associations of maternal education on birth length and length/height growth and explored the effect of adjusting for confounding factors.

Results We showed linear and positive associations of maternal education with birth length and length/height growth rates in the first four years of life. By age four, the mean height of boys in the lowest education category was 100.98 cm (SE=0.21) compared with 104.23 cm (SE=0.12) in the highest education category. The equivalent predicted heights at age four for girls were 100.08 cm (SE=0.25) and 103.00 cm (SE=0.15) in the lowest and highest education categories respectively. Thus for both boys and girls there was on average a 3 cm difference between the extreme maternal education categories. Differences in postnatal growth rates persisted in the adjusted analyses.

Conclusion Our data demonstrate an increase in the absolute and relative inequality in height after birth indicating that height inequality, which was already present at birth, widened considerably through childhood growth. These findings differ from studies in high income countries where height inequalities at birth exist but do not widen postnatally. Our results highlight the importance of postnatal environment on infant and childhood growth in a middle-income setting.