Objective To describe the cognitive, language and motor developmental trajectories of children born very preterm and to identify perinatal factors that predict the trajectories.

Design Data from a cohort of 1142 infants born at <30 weeks’ gestation who were prospectively assessed on the Bayley Scales of Infant and Toddler Development, third edition (Bayley-III) at 3, 6, 12 and 24 months corrected age, were analysed using the Super Imposition by Translation and Rotation (SITAR) growth curve analysis model.

Main outcome measures Developmental trajectory SITAR models for Bayley-III cognitive, language (receptive and expressive communication subscales) and motor (fine and gross motor subscales) scores.

Results The successfully fitted SITAR models explained 62% of variance in cognitive development, 68% in receptive communication, 53% in fine motor and 68% in the gross motor development. There was too much variation in the expressive communication subscale to fit a SITAR model. The rate of development (gradient of the curve) best explains the variation in trajectories of development in all domains. Lower gestational age, lower birth weight and male sex significantly predicted a slower rate of development.

Conclusion The rate of development, rather than single time point developmental assessment, best predicts the very preterm infant’s developmental trajectory and should be the focus for monitoring and early intervention.