Background Postnatal growth restriction is common in preterm infants and is associated with long-term neurodevelopmental impairment. Recent trends in postnatal growth restriction are unclear.

Methods Birth and discharge weights from 25 899 Californian very low birthweight infants (birth weight 500–1500 g, gestational age 22–32 weeks) who were born between 2005 and 2012 were converted to age-specific Z-scores and analysed using multivariable modelling.

Results Birthweight Z-score did not change between 2005 and 2012. However, the adjusted discharge weight Z-score increased significantly by 0.168 Z-scores (0.154, 0.182) over the study period, and the adjusted fall in weight Z-score between birth and discharge decreased significantly between those dates (by 0.016 Z-scores/year). The proportion of infants who were discharged home below the 10th weight-for-age centile or had a fall in weight Z-score between birth and discharge of >1 decreased significantly over time. The comorbidities most associated with poorer postnatal growth were medical or surgical necrotising enterocolitis, isolated gastrointestinal perforation and severe retinopathy of prematurity, which were associated with an adjusted mean reduction in discharge weight Z-score of 0.24, 0.57, 0.46 and 0.32, respectively. Chronic lung disease was not a risk factor after accounting for length of stay.

Conclusions Postnatal, but not prenatal, growth improved among very low birthweight infants between 2005 and 2012. Neonatal morbidities including necrotising enterocolitis, gastrointestinal perforations and severe retinopathy of prematurity have significant negative effects on postnatal growth.