AbstractObjectiveTo examine tracking of body size among children participating in the Norwegian Mother and Child Cohort Study (MoBa) from birth to 7 years of age and additionally to explore child and parental characteristics associated with maintenance of a high body size in this period of life.DesignAnthropometric data at birth and at 1, 3 and 7 years of age were collected by questionnaires addressed to the mother.SettingParticipants were recruited from all over Norway during the period 1999–2008.SubjectsA total of 3771 children had complete anthropometric data at birth and at 1, 3 and 7 years of age; the sample includes children born between 2002 and 2004.ResultsCohen’s weighted kappa pointed to fair (0·36) to moderate (0·43) tracking of body size from birth to 7 years of age. Generalized estimating equations further indicated that children in the highest tertile of ponderal index at birth had nearly one unit higher BMI (kg/m²) at the age of 7 years compared with children in other tertiles of ponderal index at birth. Having parents with high BMI (≥25·0 kg/m²) increased the odds of having a stable high body size from birth to 7 years of age; moreover, girls had significantly higher odds compared with boys.ConclusionsThe study indicates fair to moderate tracking of body size from birth to 7 years of age. From a public health perspective, early prevention of childhood overweight and obesity seems to be especially important among children of parents having a high BMI.