mHealth holds great promise to promote health and improve care. However, most mHealth treatments failed to achieve a significant impact on clinical outcomes, and there is surprisingly little knowledge of factors that affect mHealth effectiveness. This study examines mHealth effectiveness from a social determinant perspective. We leverage one of the world’s largest field experiments on improving the health of expectant mothers and reducing cesarean sections. We hypothesize that the husband, as one of the most significant social factors, can be an important moderator in mHealth effectiveness. Our analyses show that the husband’s healthy behavior is pivotal to enable mHealth in reducing cesarean sections. The cesarean section reduction in the strongest intervention group is 12 times bigger for those wives whose husbands exercise often that those whose husbands do not exercise. Further analyses reveal that the husband exercise behavior has a stronger influence on mHealth effectiveness when the husband has a more dominating socioeconomic status. Our findings represent one of the first studies on the critical role of social support in determining mHealth effectiveness, which has important implications to both academic research and practice of mHealth.