The paper uses a time series of repeated rounds of the General Household Survey in Great Britain to study uncertainty in fertility intentions. We show that a substantial minority of women are uncertain in their expectations about future childbearing. A comparison of reported uncertainty in GHS rounds 1979-1990 with 1991-2007 reveals that the estimated level of uncertainty is influenced by question format. At the individual level, uncertainty varies largely with demographic status and life stage-age, partnership status, parity and time since previous birth. Evidence from qualitative and quantitative studies, particularly in relation to pregnancy intentions, reinforces the reality of uncertainty about reproductive prospects. We suggest that the measurement of fertility intentions needs improvement. A new theoretical approach to fertility intentions is outlined in brief.