Babies are born with an innate drive or intrinsic motive formation with which to communicate and share meanings with others and that some authors have called intersubjectivity (S. Bråten & C. Trevarthen, 2007; C. Trevarthen, 1974, 2001). Around the ninth month of life, this motivation changes and passes from a person-to-person dyadic (primary intersubjectivity) to a person-person-object relationship (secondary intersubjectivity). S. Bråten and C. Trevarthen (2007) also proposed a third form or layer of intersubjectivity known as tertiary intersubjectivity. One hundred fifteen free-play sessions of 27 mother-child dyads (13 girls and 14 boys ages 9-37 months) were filmed and categorized using the Level of Intersubjective Attunement Scale (LISA-T; M. Pérez Burriel & M. Sadurní Brugué, 2014; M. Sadurní Brugué & M. Pérez Burriel, 2012). Results from these nine hierarchical levels are presented, following a developmental sequence or population trajectory around an interindividual variability. In this article, we propose viewing these age-related levels as windows of achievement of intersubjective milestones. The statistical analysis suggested a redesign of the LISA-T levels of intersubjectivity; thus, results from this redesign and the debate on the implications of these transitions in infant mental health development are presented.