By the time Congress passed the Family and Medical Leave Act of 1993, many employers had created maternity leave programs. Analysts argue that they did so in response to the feminization of the workforce. This study charts the spread of maternity leave policies between 1955 and 1985 in a sample of 279 organizations. Sex discrimination law played a key role in the rise of maternity leave policies. Building on neoinstitutional theory, this article explores how the separation of powers shapes employer response to law. Details of the law are often specified in administrative rulings - the weakest link in the law because they can be overturned by the courts and by Congress. Yet an administrative ruling requiring employers with disability leave programs to permit maternity leave, which employers successfully fought in the courts, was at least as effective as the identical congressional statute that replaced it. In the American context, the legal vulnerability of administrative rulings can draw attention to them, thus making the weakest link in the law surprisingly powerful.