Discussion
This study provides novel information on the effect of pre-notification letters on retention and the need for post-notifications within a 17-year prospective follow-up study of adolescents born very preterm. As hypothesized, sending pre-notifications decreased the need for post-notifications and may increase retention.It might be challenging to obtain participants’ postal or electronic addresses in prospective follow-up studies. Monetary incentive is suggested as a retention increasing method, but it is against Finnish research regulations [Retention has varied widely in previous follow-up studies of children or adolescents born very preterm [A major strength of the present study was that the CONSORT guidelines were followed accurately [