This paper describes the way in which a simple device, the pregnancy w
heel, has been used by the medical profession to impose a new way of m
easuring and experiencing pregnancy. The change involves counting in w
eeks instead of counting in months and it is gradually replacing a com
monsensical method that had deep physiological and cultural roots. In
contrast, the medical methodology of counting forty weeks is more comp
licated and lacks direct connections to the events of pregnancy. In th
e encounter between the doctor and the pregnant woman the pregnancy wh
eel has a variety of uses, among them determinations of the age and es
timated size of the fetus. It plays an additional role, however, in th
e medicalization of pregnancy by providing the doctor with privileged
information. It also influences modes of thinking through the way in w
hich it deals with the question of the beginning of pregnancy, a quest
ion that has clear moral implications.