This article examines the medical literature published in France in the period 1800--1870 on the subject of puberty and menstruation to argue that, in conjunction with the extension of school life for bourgeois girls, the period saw the emergence of a distinctive conception of feminine adolescence that predates the better--known concepts articulated in the late nineteenth century. It goes on to look beyond the scientific discourse to ask what impact this new medical understanding had on the management of girls' puberty, examining first the way it affected school practice, before using a detailed case--study of the life of Solange Dudevant to highlight the ways in which medicalised understandings of puberty and menstruation co--existed with other forms of knowledge.