BACKGROUND: Breastfeeding (BF) neonates generally lose weight after birth. Neonatal factors like gestational age, birth weight and mode of delivery can affect the neonatal weight loss after birth. Similarly, maternal age, parity and illness may contribute to newborn weight loss. However, influence of the time of birth and season on changes in weight is not well elucidated. OBJECTIVE: The aim of this study is to determine the effect of birth time and the seasonal variations on weight loss in BF newborns. METHODS: In this retrospective study of a prospectively maintained database of two sets of groups, from January 2013 to October 2016, were evaluated- Birth time group and Seasonal group; Birth time of the groups was: night time 7pm to 7am and day time 7am to 7pm; and the seasonal variations groups studies were summer, fall, winter and spring. Weight loss of >5%, 7%, and 10% at <24 hours (h), 48 h and 72 h of birth, respectively, were considered as a significant weight loss. Preterm, exclusively formula fed and neonates admitted to NICU were excluded. RESULTS: A total of 2044 newborns were analyzed. In the birth time group, babies born during the night time had significantly lost >5% of birth weight at <24 h (p < 0.01) and >7% birth weight at <48 h of life (p < 0.02). Weight loss >10% at <72 h was similar in both birth time cohorts. C-section, prolonged rupture of membrane, and maternal pre-delivery hospital stay for >12 h were significant contributing factors. Whereas, seasonal variations were not associated with weight loss in neonates. CONCLUSION: BF babies born at night time lose significantly more weight during the first two days of life and seasonal association was not found to affect weight loss in the neonates. [ABSTRACT FROM AUTHOR]