Objectives: to develop and evaluate a Middle-Range Theory for the nursing diagnosis “Disrupted Mother-Fetus Dyad Risk” in high-risk pregnancies. Methods: this methodological study was conducted in two stages: theory development and evaluation. Dorothea Orem’s General Nursing Model was used as the theoretical-conceptual foundation. Evaluation was conducted using the Delphi method with seven judges, and consensus was achieved when the Content Validity Index of the evaluated items was ≥ 0.80. Results: the theory identified 20 elements of the nursing diagnosis “Disrupted Mother-Fetus Dyad Risk” (10 risk factors, 4 at-risk populations, and 6 associated conditions), 14 propositions, and 1 pictogram. After two rounds of evaluation, the theory was considered consistent, with consensus reached for all items, each achieving a Content Validity Index ≥ 0.80. Conclusions: the Middle-Range Theory included biopsychosocial factors explaining the nursing phenomenon “Disrupted Mother-Fetus Dyad Risk,” which aids in nurses’ diagnostic reasoning.