​​CCC2000 follows all babies born in the year 2000 to mothers who at the time lived in 16 municipalities around Copenhagen. Originally, the study included 6,090 infants, representing approximately 10% of all children born in Denmark that year. The group reflects the general Danish child population in terms of health and social background, except for a higher proportion of immigrant families and slightly higher household incomes, which is typical for the Copenhagen area.

Each child in the study was identified via the unique CPR number that everyone in Denmark is assigned at birth. This number makes it possible to track the children's health and status over time through population registers, including registers with data on deaths and migration.

The families in the CCC2000 project were initially included through a quality assurance project in the municipal health care system covering infancy (0-10 months), and they were then invited to participate at key stages in their children's development: toddlerhood (1.5 years), around the start of school (5-7 years) and before and around the start of puberty (11-12 years). After this, the young people themselves were invited to participate in their mid-teens (16-17 years). Parents and children participated by answering questionnaires and taking part in interviews, as well as in surveys where they participated physically. In addition to data and information from these surveys, the researchers collected information from national registries.

Participation in the studies has generally been high when compared to other similar research projects. At the same time, there are some groups of families that have had relatively low participation. This applies first and foremost to families with a non-Danish ethnic background, families with non-cohabiting parents at the birth of the child, and families where the mothers were relatively younger, had less schooling/education and were more likely to have been treated for a psychiatric illness before the child was born.  There was also relatively lower participation among children who had received a psychiatric diagnosis themselves.

Now the children in CCC2000 have become young adults - and an important new phase of life is about to be explored.  In 2025, we invite the study population from the class of 2000 to participate in the 25-year follow-up of CCC2000 by answering questionnaires covering the most important aspects of mental and physical health in young adults.

The new follow-up involves young adults in planning the form and content of the survey and how the results will be published on the project website.​