NFBC1966 followed pregnancies with an expected delivery date in the year 1966 in the Oulu and Lapland provinces of Finland (53). A total of 5403 samples were available for analysis from NFBC1966. In NFBC1966, ‘age at first tooth’ and ‘number of teeth’ were gathered by public health professionals during the children's monthly visits to child welfare centres. ‘Age at first tooth’ was recorded as the month of visit at which the first tooth was observed (so that the first tooth could have erupted at any time between the end of the previous month and the recorded month, i.e. ‘interval censoring’). The number of teeth was recorded at 12 months. All aspects of the study were reviewed and approved by the Ethics Committee of the University of Oulu and by the respective local research committees. Participants gave written informed consent to be included in the study.
