Women in many times and places have felt pressure to bear children. But the idea of the biological clock is a recent invention. It first appeared in the late 1970s. The Clock Is Ticking for the Career Woman, the Washington Post declared, on the front page of its Metro Section, on 16 March 1978. The author, Richard Cohen, could not have realised just how inescapable his theme would become. The story of the biological clock is a story about science and sexism. It illustrates the ways that assumptions about gender can shape the priorities for scientific research, and scientific discoveries can be deployed to serve sexist ends. We are used to thinking about metaphors like the biological clock as if they were not metaphors at all, but simply neutral descriptions of facts about the human body. Yet, if we examine where the term came from, and how it came to be used, it becomes clear that the idea of the biological clock has as much to do with culture as with nature. And its cultural role was to counteract the effects of womens liberation. The role of the biological clock has been to make it seem only natural indeed inevitable that the burdens of reproducing the world fall almost entirely on women. There are moral as well as practical implications to this idea: if you do not plan your life just right, you deserve to end up desperate and alone.