This article looks at Parent-Infant Psychotherapy work with a mother with Borderline Personality Disorder and her infant son. Mother had suffered a traumatic childhood and, when her son was born, withdrew into a delusion of merger with him, avoiding what she experienced as a violent, threatening and confusing world. Drawing on psychoanalytic and attachment-mentalization theories and neuroscientific and developmental research, this article explores mother’s difficulties in acknowledging her baby as a separate being, and the impact this had on baby, who was slowly turning toward an autistic way of functioning. The article discusses the therapeutic challenges in creating a reflective, triangular space where both mother’s and baby’s experiences could be thought about and understood. Finally, the author considers an interpretative stance that includes both mother’s and baby’s shared and their separate experiences.