Genetic influence on prolonged gestation: A population-based Danish twin study☆
Section snippets
Material and methods
This study is based on three nationwide Danish registers. The Danish Twin Registry includes >65,000 twin pairs who were born in Denmark from 1870 through 1996. The ascertainment of twins who were born from 1931 through 1967 is approximately 70% complete and 99% complete for twins who were born in the latest period.11 The twins are virtually all nonadopted and reared together. The determination of zygosity was based on questionnaires concerning appearance and distinguishability. This method is
Analyses of twin similarity
The similarity in monozygotic and dizygotic twins was assessed with probandwise concordance rates, odds ratios, and correlations for prolonged gestation. The classic twin study method is based on the fact that monozygotic twins have identical genotypes, whereas dizygotic twins share, on average, one half of their genes and are no more genetically related than ordinary siblings. A greater phenotypic similarity in monozygotic twins than in dizygotic twins is to be expected, if there is a
Results
The frequency of prolonged gestation (≥42 weeks) in the study sample (Table II) was practically equal to that of the Danish population, derived from the Danish Medical Birth Register (unpublished data) of single first pregnancies of 10.2%. Monozygotic female twin pairs had a slightly higher frequency of prolonged gestations (≥41 weeks and ≥42 weeks); dizygotic male twins had a slightly lower frequency than the average for the study sample (Table II).
For all twin groups, male and female,
Comment
The study takes advantage of nationwide population-based registers, which leaves little room for selection bias. In this twin study, we investigated prolonged gestation in a population-based sample of Danish twins and found evidence for an influence of genetic factors on prolonged gestation that were inherited through the mother.
The frequency of prolonged gestation (≥42 weeks) was 9% for the Danish population as a whole in the study period.18 But because the frequency of prolonged gestation is
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