Background: Support for evolutionary care of twin or multiple birth infants whose populations are rapidly growing is of considerable interest. Co-bedding has been considered as one of the evolutionary care strategies being implemented in many neonatal intensive care units (NICUs) throughout the world. This study is designed to investigate the effect of co-bedding among premature twin or multiple birth infants on their growth and physiological stability in a NICU with the largest number of births in Iran.Methods: It is a randomized clinical trial performed on 80 pairs of premature twin infants hospitalized in the NICU of Shahid AkbarAbadi Hospital in Tehran, Iran. Patients were randomly allocated into the two groups; co-bedded and routine care. Data were collected through a demographic questionnaire and a co-bedding checklist and compared between the two groups.Results: Data analysis showed that the weight gain and mean of NICU hospitalization days in the co-bedded group were significantly different from those of the standard care infants (P<0.001). However, there was no significant difference in terms of increase in height (P=0.1), head circumference (P=0.4), heart rate (P=0.3), arterial oxygen saturation (P=0.12), and respiratory rate (P=0.68) between the two groups.Conclusion: On the basis of results, co-bedding could lead to better weight gain in twin birth infants and consequently accelerate their recovery and discharge.