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.Harmonised fertility data for the MRC National Survey of Health and Development are available under Special Licence from SN 9419. The following three data files are included:harmonised_fertility_histories_ncds: the dataset contains fertility variables derived in the NCDS at age 23, 33, 42, 46 and 50.harmonised_fertility_histories_bcs: the dataset contains fertility variables derived in the BCS70 at age 26, 30, 38, 42, 46 and 51.harmonised_fertility_histories_nextsteps: the dataset contains fertility variables derived in Next Steps at age 25 and 32.Individual identifiers are included to enable the harmonised data to be merged with the main cohort study data.

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