The Generation XXI cohort aims to identify characteristics of pregnancy and childhood that are related to development and health status in the subsequent stages of life. The study has been following over 8,600 participants born in public maternity hospitals of the Metropolitan Area of Porto, Portugal since 2005. Starting from an initial assessment at birth, the participants were then assessed at 6, 15 and 24 months, and at 4, 7, 10 and 13 years of age. At baseline, mothers had an average age of 29.7 years and 11 years of education. At age 4 years, most of the children (around 90%) had none or one sibling living with them, almost 90% lived with both parents and more than 90% were with a nanny or in a kindergarten.