Background: Gut development, function and colonisation are impaired in animal models of prematurity with intrauterine growth restriction (IUGR). The effect of Bifidobacterium breve (B. breve) supplementation on faecal bifidobacteria in small for gestational age (SGA: birth weight <10th centile due to IUGR) preterm infants is not known. Objective: We compared B. breve M-16V supplementation effect on faecal bifidobacteria in preterm (<33 weeks) SGA versus non-SGA infants in the two arms of our randomised controlled trial. Results: There were no baseline differences in the proportion of detectable B. breve counts between SGA versus non-SGA infants [probiotic: 7 (33%) versus 22 (42%), p =0.603; placebo: 1 (6%) versus 1 (2%), p =0.429]. B. breve counts did not differ between SGA and non-SGA infants in response to treatment (p = 0.589), after adjusting for baseline count (p < 0.001) and treatment allocation (p <0.001). An interaction term between growth status and treatment showed negligible change (p =0.938). Probiotic treated SGA infants reached full feeds earlier than SGA controls (HR 2.00, 95% CI 1.05–3.82, p = 0.035): Median (IQR): 16 (12–26) versus 19 (11–25) days, after adjustment for age at starting feeds and gestation <28 weeks. Conclusion: Response to B. breve M-16V supplementation was not significantly different in preterm (<33 weeks) SGA versus non-SGA infants.