Objective To assess the association between early initiation of parenteral nutrition (PN) and body growth in preterm infants with very low birth weight (VLBW). Design Causal inference analysis with confounders preselected by causal diagram based on the NeoNutriNet cohort containing data of infants born between 2011 and 2014 from 13 hospitals from 5 continents. Patients Neonates with birth weight ≤1500 g. Interventions PN initiated within the first day of life (early PN) versus within day 2–5 (delayed PN). Main outcome measures The primary outcome was body weight z-scores at postmenstrual age (PMA) 36 weeks or early discharge or death, whichever comes first (WT z-score END). Secondary outcomes included WT z-scores at week 1 and 4 of life (WT z-scores CA1 and CA4), corresponding growth velocities (GVs), mortality and incidence of necrotising enterocolitis (NEC), and duration and episodes of antibiotic treatment. Results In total, 2151 infants were included in this study and 2008 infants were in the primary outcome analysis. Significant associations of early PN were found with WT z-score END (adjusted mean difference, 0.14 (95% CI 0.05 to 0.23)), CA4 (β, 0.09 (0.04 to 0.14)) and CA1 (0.04 (0.01 to 0.08)), and GV PMA 36 weeks (1.02 (0.46 to 1.58)) and CA4 (1.03 (0.56 to 1.49), all p<0.001), but not with GV CA1 (p>0.05). No significant associations with mortality, incidence of NEC or antibiotic use was found (all p>0.05). Conclusions For VLBW infants, PN initiated within the first day of life is associated with improved in-hospital growth.