Background: The worldwide reduction in fertility, change in demographic policies, rise in divorce ages, urbanization, and growth of the nuclear family require greater attention. Having an only child in the context of child-rearing seems to be a strategy for creating a balance between individual, marital, family, and environmental conditions, leading Iran to become an aging country in the future. Methods: This qualitative study is based on semi-structured interviews with 11 married women with only one child. Results: We used the grounded theory method to investigate the factors influencing Iranian mothers\\\\\\\' decision to have an only child. We extracted 571 initial concepts, 55 subcategories, and 23 main categories by coding the interviews. The findings are classified into five categories shaping the final theoretical model. The results show that people can fall into groups, \\\\\\\"forced only-child\\\\\\\" and \\\\\\\"voluntary only-child,\\\\\\\" regarding their attitudes toward having an only child. Conclusion: Participants\\\\\\\' decision to have an only child is influenced by external factors related to society\\\\\\\'s economy, politics, and culture. On the other hand, this decision also reflects the individuals\\\\\\\' circumstances shaped by internal factors that impact the participants. Furthermore, maintaining the decision is influenced by factors that directly affect the causal conditions and ultimately result in having an only child.

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