The transition to motherhood in women with obesity is a complex and challenging process for comprehensive health care, and requires rethinking the theoretical and conceptual foundations that guide the Nursing professional work. This study sought to develop a situation-specific nursing theory for the care of women with obesity in the process of transitioning to motherhood. Grounded theory was employed through the application of a constant process of interactive and integrative comparison of data obtained from the documentary review, the investigative findings, and the theoretical foundations by Im and Meleis. The results demonstrate the development of a situation-specific theory of the transition to motherhood in women with obesity, where obstetric fatphobia is proposed as a key conceptual axis to understand this phenomenon. It is concluded that the “Conflictive matrescence” is a situation-specific, prescriptive theory derived from multiple sources, which describes the physical, psychological or emotional and social/relational changes experienced by women who are obese and go through motherhood. The ambivalent relationship between health risk and the desire to be a mother is recorded, as well as the change in expectations and the processes of emancipation and resistance against weight stigma and fatphobia.