Compliance to regulations and regulatory approval are requirements for many medical device software systems. In this paper, we investigate the implications of medical device software regulations to the design of software systems. We do so by focusing on the American and European regulatory authorities and review the legal requirements for regulatory approval of medical devices. We define a simplified process for regulatory approval, consisting of five steps, and enhance this process by descriptions of how to decide whether a software system is a medical device and how to identify the class of the device. Moreover, we review software modularity in the implementation of software medical device and propose a set of preliminary principles for architectural design of software medical device based on a set of constrains identified from the reviewed regulations.