The Avon Longitudinal Study of Parents and Children (ALSPAC): an update on the enrolled sample of index children (the G1 cohort) in 2025.

The Avon Longitudinal Study of Parents and Children (ALSPAC) is an ongoing prospective population-based study that has been running for almost 35 years. Pregnant women with expected dates of delivery falling between 1 st April 1991 and 31 st December 1992 were recruited and the health and development of the index children from these pregnancies and that of their family members have been followed ever since. At the time of enrolment, the sampling frame for those eligible to enrol was constructed retrospectively using linked recruitment and health service records. Further explicit recruitment drives took place at the ages of 7 and 18 years to enrol G1 participants (the index children) that were eligible but did not take part originally. These were supplemented by opportunistic contacts from the age of 7 resulting in additional participants being enrolled. Around the age of 30, a further concerted effort was undertaken to encourage unenrolled but eligible participants to take part in the study's '@30' clinic. This data note updates the status of recruitment of the index children in 2025, following the completion of the clinic. In total, 1014 additional G1 participants have been enrolled in the study since the age of 7 years, with 101 of these in the most recent phase of recruitment that has taken place since the age of 24. At the time of writing ALSPAC now has a baseline cohort of 15,002 G1 participants alive at 1 year of age.