Background: Social support facilitates a woman's transition to motherhood. This major developmental transition can be stressful as it includes adaptation of self as well as learning new infant care practice skills. Although a number of instruments have been developed to measure social support, none have been developed or underpinned by theory in the context of perinatal infant care practices. Aim: To develop a reliable and valid instrument to measure social support for new mothers in the perinatal period. Methods: Phase 1 involved the development of instrument structure and content. Constructs to be measured were defined through an analysis of relevant theoretical and empirical literature. Phase 2 established the psychometric properties of the functional domain of the PICSS. Exploratory factor analyses and principal Component Analyses were undertaken with a sample of first-time mothers (n = 371) from postnatal wards of a large maternity hospital. Item reduction and Cronbach's alpha reliability tests were performed. The structural social support domain was not amenable to psychometric testing. Results: Exploratory Factor Analyses and Principal Component Analyses of the functional domain resulted in a logically coherent 19-item, two-factor solution. The first factor ‘Supporting Presence’ has nine items (Cronbach's alpha = 0.90) and the second factor ‘Practical Support’ has ten items (Cronbach's alpha = 0.86). Conclusions: The PICSS is a coherent and valid measure of social support for new mothers in the postnatal period in the context of infant care practices.