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. To develop a reliable and valid instrument to measure social support for new mothers in the perinatal period. 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. 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). 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. •The PICSS is a coherent and valid measure of social support for new mothers in the postnatal period.•The first social support instrument that was developed underpinned by theory in the context of the perinatal period.•The PICSS can be used to assess for maternal social support needs in the postnatal period.