The FinnGeDi study aims to investigate different aspects of Gestational Diabetes Mellitus (GDM) as diagnosed by comprehensive screening, which is expected to increase the prevalence of GDM by identifying previously undiagnosed cases. The study consists of two arms: a prospective clinical, genetic case-control arm and a national register-based arm, both based in Finland. The case-control arm includes over 1,000 mothers who had GDM during pregnancy and over 1,000 mothers in a non-GDM control group, their children and the fathers of their children. Pregnant women were recruited from maternity units across seven hospitals in Finland at the time of their child’s birth. Baseline data, including questionnaires, medical data from hospital records and baseline register data, were collected at the time of enrolment from 2009 to 2012. Participants have been followed up with linked registry data. An active follow-up of a subset of participants began in 2023 and is due to be completed at the end of 2025.