In spite of widespread awareness of the risk factors for sudden unexpected death in infancy (SUDI), many infants continue to be exposed to a range of risks, and most deaths now occur in situations where risk reduction measures have not been followed. SUDI is also more likely to occur in families with identified vulnerabilities such as young maternal age, low educational achievement, smoking, alcohol and substance use. Families with these characteristics can be described as having a higher risk for SUDI and sudden infant death syndrome (SIDS). This study adopts a qualitative approach using Interpretative Phenomenological Analysis (IPA), to gain an understanding about the lived experience of young first-time mothers identified as being at increased risk for experiencing SUDI, their understanding of safe sleep practices, what factors influence their decision-making and behaviour in relation to their infant's sleep environment, and whether infant-care practices change over time. A homogeneous sample of five first-time mothers, with identified characteristics known to increase the risk of experiencing SUDI, were recruited antenatally. Serial in-depth interviews were conducted during the antenatal and postnatal period. The interviews were transcribed, and data analysed to identify emergent, subordinate and superordinate themes. Superordinate themes of transition, the construction of knowledge, and fractured application, revealed that adolescence was the context for understanding how information shared with them was received and processed, and ultimately how that knowledge was translated and applied. Filiano and Kinney’s (1994) triple-risk hypothesis has been enhanced to explain the emergent theories generated from this inductive research, adding a new domain of ‘social vulnerability’. This new domain recognises the unique contribution that this exploratory research makes in providing a deeper understanding of the lived experience of vulnerable young mothers; and how this context impacts on their decision-making with regard to infant-care practices, and how this may increase the risk of SIDS to their infant in the sleep environment.