The Harmonised Data in Five National Longitudinal Cohort Studies project, supported by the Economic and Social Research Council (ESRC), brings together data from five British cohort studies: the 1946 National Survey of Health and Development (NSHD), the 1958 National Child Development Survey (NCDS), the 1970 British Cohort Study (BCS70), Next Steps (formerly the Longitudinal Study of Young People in England), and the Millennium Cohort Study (MCS). NCDS, BCS70, Next Steps, and MCS receive core funding from the ESRC and are hosted by the Centre for Longitudinal Studies at UCL. NSHD is funded by the Medical Research Council (MRC) and is hosted by the Unit for Lifelong Health and Ageing at UCL. The Fertility Harmonisation project aimed to enhance data in these four British cohort studies (NSHD, NCDS, Next Steps and BCS70) through retrospective harmonisation of fertility information. This retrospective harmonisation serves two primary purposes: improving the measurement of fertility (as an outcome, predictor, or control variable) within cohort analyses, and facilitating cross-cohort research on fertility using these rich datasets.Separate longitudinal datasets were created for each cohort, covering survey sweeps from early adulthood to the early fifties, marking the end of the reproductive window for most cohort members. Derived variables provide a summary of fertility (live births) at each survey sweep, including whether the cohort member had children, the number of children, the age of the eldest and youngest child, and the number of boys and girls. The focus was exclusively on live births, excluding pregnancies that were terminated, miscarriages, or stillbirths.SN 9419 includes harmonised fertility data for the MRC National Survey of Health and Development only. Harmonised fertility data for NCDS, BCS70 and Next Steps are available under Safeguarded data access from SN 9418. The dataset contains fertility variables derived in the NSHD at age 19, 20, 22, 26, 31, 36, 43 and 53.The research IDs differ from the original research IDs and data can therefore not be merged with other NSHD data files. Access to the harmonised variables in connection with other NSHD variables can be requested via the usual NSHD data sharing request.