Purpose: To assess gender differences among residents regarding their plans to have children during residency and determine the most influential reasons for these differences. Method: Using the Health Belief Model as a framework, the authors created an instrument to survey 424 residents from 11 residency programs at three academic medical institutions about their intentions to have children during residency. The authors developed a scale to assess the perceived career threats of having children during residency, evaluated its psychometric properties, and calculated the effect of the mediators. Results: The response rate was 77% (328/424). Forty-one percent of men versus 27% of women planned to have children during residency (P = .01). The instrument measured four career threats-extended training, loss of fellowship positions, pregnancy complications, and interference with career plans-on a five-point Likert scale. The scale had a Cronbach alpha of 0.84 and an eigenvalue of 2.2. Compared with men, women had higher scores for each item and a higher mean score (2.9 versus 2.1, P = .001), signifying greater belief in the potential of pregnancy to threaten careers. After adjusting for age, institution, postgraduate year, and knowledge of parental leave policies, women were less likely to plan to have children during residency (odds ratio 0.46 [95% confidence interval 0.25-0.84]). In mediation analysis, threats to career explained 67% of the gender variance. Conclusions: Women residents intentionally postpone pregnancy because of perceived threats to their careers. Medical educators should be aware of these findings when counseling female trainees. Copyright © by the Association of American Medical Colleges.