Background and Objective: In recent decades, the family has undergone profound transformations that have altered patterns of marriage and childbearing, leading to delayed marriage. Previous research has predominantly focused on economic and structural factors, with less attention paid to the role of lifestyle changes through an integrated approach. This study aims to investigate the impact of lifestyle changes on family demographic patterns in the city of Boroujerd.Method: This qualitative research was conducted using a grounded theory approach. Data were collected through in-depth interviews with 15 married and soon-to-be-married participants in Boroujerd and analyzed using open, axial, and selective coding.Findings: The core phenomenon of "fundamental reflexivity on the desirability of the large-family structure" emerged under causal conditions including value transformation, changing women's roles, economic uncertainty, and consumerism. This process occurred in the context of urbanization and changes in kinship relations, intensified by intervening conditions such as population policies, technology, and media. Participants adopted strategies of strategic delay, planned one-child policy, prenuptial agreements, and relationship quality enhancement, leading to fertility decline, population aging, and contractualization of the family.Conclusion: Lifestyle transformations in Boroujerd have altered the traditional family and childbearing patterns, making childbearing a more conscious decision based on the assessment of costs and consequences. Under these circumstances, quality of life and child-rearing have taken priority over increasing the number of children. Therefore, addressing demographic changes requires multidimensional policies aligned with the cultural, value-based, and lifestyle transformations of families.