In recent years, economic and social conditions in Tehran have made many families to increasingly reflect on and raise questions about childbearing. The primary aim of this research is to explore the role of economic and social rationality in childbearing behaviour among families in Tehran using grounded theory approach. This study also deals with questions of “how Tehranian families define economic and social rationality in relation to their childbearing behaviour?, and “How do they react to their interpretation of childbearing in reality?” . To answer these questions, the qualitative research method, using grounded theory approach was employed. A combination of theoretical, purposive and snowball sampling was used to collect the sample. In-depth interviews were conducted with 89 married men and women aged 15-49 years(60 women and 29 men). The results showed that, the feeling economical and social insecurity and also the growing costs of children creates some grounds to  define childbearing as a long-term commitment and binding responsibility and families seek to withdraw from an irreversible decision. The conditions lead Tehranian families emphasize quality rather than quantity and thus opted for low fertility as a rational behaviour. The implication of these results is that policy makers and planners need to apply the mechanisms of economic stability and sustainable occupation.