In recent times, parents have become increasingly concerned, both subjectively and objectively, about their adolescents' body height/weight growth. Parent-adolescent interactions about this issue and the potential socio-psychological consequences of such interactions should be considered as an important influencing factor on the future of adolescents' sexual and reproductive health. To achieve a greater understanding of such concerns, it is necessary to further elucidate parents' experiences on this topic, so as to expand the existing literature. This study aimed to explain the perceptions of parents' concerns regarding their adolescents' growth characteristics in the socio-cultural context of Iran as a transitional society. This paper is part of a larger qualitative study designed using the Constructivist Grounded Theory Methodology (CGTM). We conducted open-ended intensive interviews with eleven parents individually and recruited them through purposeful and theoretical sampling from a teaching hospital, community, and a primary school in Tehran with theoretical sampling variation in terms of teenagers' age, sex, and birth order, place of residence, parents' occupation and education, and the self-reported socio-economic status. Using the analytical procedures of the CGTM, we performed analyses. In the findings, the concept of 'living with constant sense of uncertainty' emerged from the subcategories including 'feeling existing and potential concern about expected minimum and maximum bio-positions of growth,' 'feeling potential concern about biological health consequences,' 'feeling potential concern about the emergence of early/late maturity signs,' 'feeling potential concern about adolescent's emotional threat,' 'feeling concerned about future employment, education, marriage, and fertility,' and 'feeling potential concern about the society's view'. These findings suggest that parents are living with a constant sense of uncertainty about their teens' growth characteristics throughout the transition from adolescence. All stakeholders including parents, health-care practitioners and policymakers, and anthropologists/sociologists should be focus on such concerns, in order to manage them and their possible socio-psychological burdens.