Critical thinking by health professionals has been linked to improved public health outcomes due to its effect on evidence-based practice, error rates, use of technology, and patient centered care. To ensure the best maternal newborn health outcomes, critical thinking in nurses is an important goal. The major purpose of this research was to examine the degree to which topic knowledge, individual interest, and relational reasoning predict critical thinking in maternity nurses. For this online study, 182 maternity nurses were recruited from national nursing listservs. A key finding was that the three individual difference factors explained a significant proportion of variance in critical thinking with a large effect size, with topic knowledge identified as the strongest predictor. Notably, individual interest and relational reasoning, which are not strongly emphasized in nursing education, were identified as significant predictors of critical thinking. The findings suggest that these individual difference factors should be included in future studies of critical thinking in nursing.