The study was started by professor Paula Rantakallio in the two Northernmost provinces in Finland (Oulu and Lapland) already in the year 1965 when the mothers were pregnant. Data on the individuals born into this cohort was collected since the 24th gestational week as well as their mothers and, to a lesser extent, fathers. The cohort included 12055 mothers and they had 12068 deliveries (13 women delivered twice).
Cases belonging to survey were determined by the calculated term. A small percentage of the births occurred towards the end of 1965 and early in 1967. The calculated term, as was customary at that time, was counted from the first day of the last menstrual period. Where this date was unknown the expected term was estimated from the date of commencement of foetal movements and progress of the pregnancy. The study covered all live born and stillborn infants with birth weight of 600 grams or more.
According to the Finland's central Office of Statistics, births in the study area during 1966 totalled 12527, so study population comprised 96.3 per cent of all births during 1966 in that area.
Altogether 12231 children were born into the cohort, 12058 of them live-born. The original data have been supplemented by data collected with postal questionnaires at the ages of 1, 14 and 31 years and various hospital records and national register data.
Last updated: 20.1.2017