Drawing on a social identity framework of mathematical development, the authors present a model, Improving Girls' Math Identity (IGMI), designed to address two key "leaks" in the female STEM pipeline: undergraduate and middle school. IGMI involves a supportive professional development network for undergraduate women preparing to transition into mathematics-related careers, and a mentorship program connecting middle school girls with these undergraduates to develop metacognition, problem-solving, and spatial skills. Preliminary evidence demonstrates that the model successfully strengthens undergraduates' problem-solving abilities, improves resilience and persistence in pursuing STEM fields, and increases investment in supporting future cohorts of women in mathematics and science.