The Little in Norway (LiN) project is a cross-disciplinary prospective longitudinal study starting in pregnancy. It was set up to investigate maternal and paternal mental health functioning in the transition to parenthood, detect pathways to healthy and aberrant child development and generate new knowledge about mechanisms underlying differential child mental health susceptibility.The LiN cohort is a community-based sample comprising 1036 families (1036 mothers, 884 partners, 1017 children). All pregnant women and their partners receiving routine prenatal care at well-baby clinics at nine geographically selected sites across Norway were invited to participate. Enrolment took place from September 2011 to October 2012. This cohort profile comprises 10 data collection waves spanning from enrolment in pregnancy until child age 18 months.Four types of information have been collected: multi-informant questionnaire reports, direct observation of interaction, test data and biological samples. The most significant findings so far relate to three domains of results. First, when examining risk factors for parental mental health problems, results showed that the parents' own adverse childhood experiences and attachment style were related to anxiety, depression and stress in the perinatal period. The perception of difficult child temperament was also found to contribute to parenting stress in the first year after birth. Second, we studied how parental mental health risk factors were related to later child development and social emotional functioning, for example, linking maternal symptoms to social-emotional outcomes and paternal symptoms to language outcomes. Third, we investigated the relation between maternal nutrition during pregnancy and aspects of early child development. Results showed that mild to moderate maternal iodine deficiency in pregnancy was associated with poorer language skills up to 18 months, but not with reduced cognitive or fine and gross motor skills.A data collection point at 36 months is completed and currently being analysed. A new data collection wave is planned when the children are 8 years of age.ISRCTN66710572.