Describing differences in weight and length growth trajectories between white and Pakistani infants in the UK: analysis of the Born in Bradford birth cohort study using multilevel linear spline models

Objective: To describe the growth pattern from birth to 2 years of UK-born white British and Pakistani infants. Design: Birth cohort. Setting: Bradford, UK. Participants: 314 white British boys, 383 Pakistani boys, 328 white British girls and 409 Pakistani girls. Main outcome measures: Weight and length trajectories based on repeat measurements from birth to 2 years. Results: Linear spline multilevel models for weight and length with knot points at 4 and 9 months fitted the data well. At birth Pakistani boys were 210 g lighter (95% CI −290 to −120) and 0.5 cm shorter (−1.04 to 0.02) and Pakistani girls were 180 g lighter (−260 to −100) and 0.5 cm shorter (−0.91 to −0.03) than white British boys and girls, respectively. Pakistani infants gained length faster than white British infants between 0 and 4 months (+0.3 cm/month (0.1 to 0.5) for boys and +0.4 cm/month (0.2 to 0.6) for girls) and gained more weight per month between 9 and 24 months (+10 g/month (0 to 30) for boys and +30 g/month (20 to 40) for girls). Adjustment for maternal height attenuated ethnic differences in weight and length at birth, but not in postnatal growth. Adjustment for other confounders did not explain differences in any outcomes. Conclusions: Pakistani infants were lighter and had shorter predicted mean length at birth than white British infants, but gained weight and length quicker in infancy. By age 2 years both ethnic groups had similar weight, but Pakistani infants were on average taller than white British infants.