Approximately 15% of infants worldwide are born with low birthweight (<2500 g). These children are at risk for growth failure. The aim of this umbrella review is to assess the relationship between infant milk type, fortification and growth in low-birthweight infants, with particular focus on low- and lower middle-income countries. We conducted a systematic review in PubMed, CINAHL, Embase and Web of Science comparing infant milk options and growth, grading the strength of evidence based on standard umbrella review criteria. Twenty-six systematic reviews qualified for inclusion. They predominantly focused on infants with very low birthweight (<1500 g) in high-income countries. We found the strongest evidence for (1) the addition of energy and protein fortification to human milk (donor or mother's milk) leading to increased weight gain (mean difference [MD] 1.81 g/kg/day; 95% confidence interval [CI] 1.23, 2.40), linear growth (MD 0.18 cm/week; 95% CI 0.10, 0.26) and head growth (MD 0.08 cm/week; 95% CI 0.04, 0.12) and (2) formula compared with donor human milk leading to increased weight gain (MD 2.51 g/kg/day; 95% CI 1.93, 3.08), linear growth (MD 1.21 mm/week; 95% CI 0.77, 1.65) and head growth (MD 0.85 mm/week; 95% CI 0.47, 1.23). We also found evidence of improved growth when protein is added to both human milk and formula. Fat supplementation did not seem to affect growth. More research is needed for infants with birthweight 1500-2500 g in low- and lower middle-income countries.