Twin study

Data of twins and their controls

Number of cases
Data obtained: N = 652

Data collection
The data were collected from a geographically defined area of Northern Finland, the provinces of Oulu and Lapland. The survey was started during pregnancy, and included all cases with an expected data of delivery in 1966. The investigation at the antenatal clinics covered 96% of all deliveries in the area in 1966 (Rantakallio, 1969). Single births numbered 11,905 and twin births 163, including 11,744 live-born single infants and 314 twins (153 whole twin pairs, 8 twins whose co-twin was stillborn and two stillborn pairs). 11,623 singletons and 289 twins were still alive at 28 days of age, the latter figure including 144 of the first-born twins and 145 of the second-born ones. Perinatal mortality was thus 2.4% for single deliveries and 11.3% for twins. At the age of 14 years, 286 twins were alive, of which one pair had moved to Australia and had no further contact with the research group. After exclusion of these two children, the final group consisted of 284 twins. Each was assigned two control children from the cohort. The first control child was matched only by sex, mother's place of residence during the pregnancy and mother's age and parity, since the chances of having twins are known to increase with maternal age and parity. Because of the higher perinatal morbidity of twins, each twin was assigned a second control child who was matched by the following factors: sex, mother's age and parity, birth weight, gestational age, intrauterine growth (possible growth retardation in the same percentile as the twin) and occurrence of perinatal asphyxia, hyperbilirubinaemia and hypoglycemia.

Information concerning the deliveries and the condition of the neonates was gathered from the maternity hospitals and specialized neonatal departments in Northern Finland. Follow-up data on the growth and physical and mental development of the children were collected at various ages and from various sources, the latest follow-up having been in 1985. Representing an age of 19 years (Rantakallio, 1988). Information on growth and early development was gathered by health nurses at the children's welfare clinics at the age of one year and later information concerning the children's in by the parents and the children themselves at the age of 14 years.

Upon completing the compulsory nine classes of the primary and secondary school at the age of 16 years, all educable adolescents in Finland are advised to take part in the national application system which arranges entrance to almost all forms of intermediate-level education, including the upper secondary schools and vocational schools but excluding certain short courses arranged by employment agencies or private employers. Applications are sifted by committees in the provincial educational offices and passed on to individual schools, which perform their final selections. Some adolescents take part in this national application system more than once: 53% once, 34% twice, 7% three times and stored in national registers, data from which were added to the records of the present birth cohort in the years 1981-85. Because of the lack of national registers for the outcome of this intermediate-level education, information on the outcomes concerning the twins and both groups of controls was obtained from the offices of these schools.

Main articles
Moilanen I., Rantakallio P.: The growth, development and education of Finnish twins: A longitudinal follow-up study in a birth cohort from pregnancy to nineteen years of age. Growth, Development & Aging 1989, 53: 145-150.

Last updated: 9.7.2020