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Familial correlations in birth weight and gestational age have been explained by fetal and maternal genetic factors, mainly in studies on offspring of twins. The aim of the present intergenerational study was to estimate and compare fetal and maternal genetic effects and shared sibling environmental effects on birth weight and gestational age and also on crown-heel length and head circumference. The authors used path analysis and maximum likelihood principles to estimate these effects and, at the same time, to adjust for covariates. Parent-offspring data were obtained from the Medical Birth Registry of Norway from 1967 to 2004. For the analysis of birth weight and crown-heel length, 101,748 families were included; for gestational age, 91,617 families; and for head circumference, 77,044 families. Assuming no cultural transmission and random mating, the authors found that fetal genetic factors explained 31% of the normal variation in birth weight and birth length, 27% of the variation in head circumference, and 11% of the variation in gestational age. Maternal genetic factors explained 22% of the variation in birth weight, 19% of the variation in birth length and head circumference, and 14% of the variation in gestational age. Relative to the proportion of explained variation, fetal genes were most important for birth length and head circumference. [ABSTRACT FROM AUTHOR]

Background In a number of studies, birthweight has been associated with cognition and educational attainment into adult age. However, the association is not clear between birthweight and work participation in adulthood. We investigated this association assessing to which extent it was influenced by circumstances concerning family background or disease in early life.Methods Through linkage between several national registers containing personal information from birth into adult age we established a longitudinal, population-based cohort study. Study participants were all 308 829 singletons born in Norway in 1967–1971 as registered by the Medical Birth Registry of Norway who were national residents at age 29. The study outcome was unemployment defined as a lack of personal income among people who were not under education in the calendar year of their 29th birthday as registered by the National Insurance Administration and Statistics Norway.Results Birthweight below the standardized mean was associated with unemployment. The risk of unemployment increased by decreasing birthweight for both women and men and also after adjustment for potential confounding factors. The association was evident both in people with or without social disadvantage, as well as people with or without childhood disease. Still, birthweight below the standardized mean explained much less of the unemployment risk than did social disadvantage (attributable fractions 8.0% versus 28.3% for women and 10.0% versus 40.2% for men).Conclusion Birthweight below the standardized mean was independently associated with unemployment at age 29, also in the normal birthweight range. [ABSTRACT FROM AUTHOR]

Background Caesarean section rates are increasing. It is not clear whether a familial predisposition to caesarean section exists. Methods Births registered in the Medical Birth Registry of Norway during 1967–2005 were linked to construct 440 236 grandmother–parent units and 275 001 same sex full sibling units. Log-binomial regression models were used to explore associations. Results A mother born by caesarean section had a 55% increased risk of having her first child by caesarean section [adjusted relative risk (RR) 1.55, 95% CI 1.48–1.62]. The same was not found for fathers born by caesarean section (RR 1.02, 95% CI 0.96–1.10). A younger sister, whose older sister had her first child by caesarean section, had a 45% increased risk of having her first child by caesarean section (RR 1.45, 95% CI 1.40–1.51). If an older brother had his first child by caesarean, this did not constitute an increased risk for the younger brothers partner (RR 1.02, 95% CI 0.97–1.08). Similar associations were found in the subgroups of obstetric low risk deliveries. Conclusions A female-to-female familial predisposition to caesarean section was observed. It could be caused by biologic inheritance, primarily working through maternal alleles and/or environmental factors. The results imply that both mechanisms could be important. [ABSTRACT FROM AUTHOR]