How we conceptualise and seek to manage time in relation to pregnancy and birth is always socially and culturally shaped and continues to play a central role in reproductive experiences and in the working patterns and lives of maternity care providers. Time is measured, marked and managed from the first signs of pregnancy to the postnatal period and ‘watching the clock’ has played a central role in conceptualising and managing risk in maternity care. In this chapter, we focus particularly on concepts and practices relating to gestational length and shifts in how the duration of pregnancy is counted and (increasingly) limited in an effort to manage risk. Through critical analysis of research and practice, we suggest that increasing intervention to control the duration of pregnancy is related to the imperative to control the uncertainty inherent in pregnancy and birth by anticipating and seeking to prevent future possible risks.