Aims The aim of this study was to determine if syndrome-specific birth weight charts were beneficial for babies with Down syndrome in England and Wales.

Methods Birth weights of 8825 babies with Down syndrome born in England and Wales in 1989–2010 were obtained from the National Down Syndrome Cytogenetic Register. Birth weight centiles for 30–42 weeks gestation by sex were fitted using the LMS method and were compared to those for unaffected babies from the UK-WHO growth charts.

Results For babies born with Down syndrome the median birth weight from 37 to 42 weeks was 2970 g (10th-90th centile: 2115–3680) for boys and 2930 g (2100–3629) for girls, and the modal age of gestation was 38 weeks, 2 weeks earlier than for unaffected babies. At 38 weeks gestation they were only slightly lighter than unaffected babies (159 g for boys and 86g for girls). However at 40 weeks gestation the shortfall was much greater (304 g and 239 g respectively).

Conclusion In neonates with Down syndrome there is little evidence of growth restriction before 38 weeks gestation, so up to this age it is appropriate to use the UK-WHO birth weight charts. Thereafter birth weight is below that of unaffected babies and it should be plotted on the UK Down syndrome growth charts.