Objective: To evaluate the influence of the severity of prematurity based on birth weight on maternal distress and sensitivity and on infant development. Methods: Sixty-eight mothers and their preterm babies (30 babies classified into Extremely-Low-Birth Weight-ELBW and 38 into Very-Low-Birth Weight-VLBW) were assessed at 9 months of infant corrected age, using: Griffiths Scales for infant development, CARE-Index for maternal sensitivity during 5-minute of mother-infant interaction, and Parenting Stress Index-Short Form (PSI-SF) for maternal distress. Sixty-six healthy full-term infants (FT) and their mothers were assessed with the same procedure. Results: ELBW, VLBW and FT groups showed similar levels at CARE-Index and PSI-SF. Nevertheless, considering infant development as outcome, a significant interaction between birth weight and maternal distress emerged, with higher Hearing & Language mean quotients in association with Non-Distressed mothers, but only in VLBW infants, compared to FT ones. Also the interaction between birth weight and maternal sensitivity influenced infant development: higher quotients (Eye-hand coordination, Hearing & Language, Locomotor) were significantly associated with sensitive mothers but only in ELBW infants. Conclusion: The severity of prematurity, in interaction with the degree of maternal distress and sensitivity, influenced the level of infant development. Practical implications: Taken together, these results suggest the relevance of considering severity of prematurity and maternal variables in order to implement appropriate interventions for supporting parenting role after a pre-term birth and promoting an adequate infant development. (C) 2017 Elsevier Ireland Ltd. All rights reserved.