Abstract: Knowledge about child development (KIDI) is an important dimension of infant care. Although it has been discussed that adolescent mothers, compared with adult mothers, reveal lower average of KIDI, little is known about variables associated with it, as well as its variation over time. We aimed to investigate longitudinally KIDI during infant’s first year and its interaction with maternal behaviors, social support/ postpartum depression. KIDI was evaluated according to its corrected and wrong answers for each dimension of infant’s development. Participants were originally 49 adolescents (16.49 years; SD = 1.58). They answered to KIDI, social support, and maternal depression at pregnancy, 3 (n = 41), 6 (n = 39), and 12 months postpartum (n = 35). Results revealed that KIDI increased during the first year postpartum, qualified with an interaction with social support. Social support plays an important role to increase KIDI, particularly within adolescent mothers living in vulnerable backgrounds.