Below replacement fertility (<2.1 children) appeared since 2006 in Iran which has generated many concerns. Below replacement fertility represent some rise in single child fertility behavior. Accordingly, main question of this study is that “what is interpretation and understanding of men and women resided in Tehran on rationales and motivations of having one child”. In addition, “what underlying factors are responsible for such decision?” To answer this question a number of 14 semistructured in depth interviews were conducted with men and women who are about to marry, and those who were married with no children, married people who had one child younger than 5 years of age and those who were at the end of their reproductive age and had single child and resided in Tehran. Theoretical underpinning of the questions derived from the Theory of Planned Behavior, Second demographic Transition, and Social Cognitive Theory. In depth interviews were coded openly using conventional content analysis. Then, based on manifest common meaning and constant comparison method, open codes were categorized into main categories and subcategories. Results showed that decision to have one child is in fact “a strategy for making a balance between all circumstances at the individual, conjugal, family and environmental levels”. This decision is the outcome of an interaction between “sum of attitudes, feelings, and skills in favor of having single child”, “unsupportive domain of conjugal interaction for childbearing”, “unsupportive domain of extended family for childbearing”, and finally “unsupportive domain of society for childbearing”. Hence, in line with pronatalist policies, complexity of such decision and behavior is necessary to be considered. According to these findings, pronatalist Interventions should consider enhancement of occupational security for employed women, promotion of quality of childcare facilities, changing the norms of ideal number of children, enhancement of sense of social and economic security in the family and enhancement of family relationship.