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Although there is increasing evidence that genetic factors influence gestational age, it is unclear to what extent this is due to fetal and/or maternal genes. In this study, we apply a novel analytical model to estimate genetic and environmental contributions to pregnancy history records obtained from 165,952 Swedish families consisting of offspring of twins, full siblings, and half-siblings (1987–2008). Results indicated that fetal genetic factors explained 13.1% (95% confidence interval (CI): 6.8, 19.4) of the variation in gestational age at delivery, while maternal genetic factors accounted for 20.6% (95% CI: 18.1, 23.2). The largest contribution to differences in the timing of birth were environmental factors, of which 10.1% (95% CI: 7.0, 13.2) was due to factors shared by births of the same mother, and 56.2% (95% CI: 53.0, 59.4) was pregnancy specific. Similar models fit to the same data dichotomized at clinically meaningful thresholds (e.g., preterm birth) resulted in less stable parameter estimates, but the collective results supported a model of homogeneous genetic and environmental effects across the range of gestational age. Since environmental factors explained most differences in the timing of birth, genetic studies may benefit from understanding the specific effect of fetal and maternal genes in the context of these yet-unidentified factors. [ABSTRACT FROM AUTHOR]

Context: While the nature of common fears changes over development, we do not know whether genetic effects on fear-proneness are developmentally stable or developmentally dynamic., Objective: To determine the temporal pattern of genetic and environmental effects on the level of intensity of common fears., Design: Prospective, 4-wave longitudinal twin study. Structural modeling was performed with Mx., Setting: General community., Participants: Two thousand four hundred ninety twins and their parents from the Swedish Twin Study of Child and Adolescent Development., Main Outcome Measure: The level of parent- and/or self-reported fears obtained at ages 8 to 9, 13 to 14, 16 to 17, and 19 to 20 years., Results: Thirteen questionnaire items formed 3 distinct fear factors: situational, animal, and blood/injury. For all 3 fears, the best-fit model revealed developmentally dynamic effects and, in particular, evidence for both genetic attenuation and innovation. That is, genetic factors influencing fear intensity at age 8 to 9 years decline substantially in importance over time. Furthermore, new sets of genetic risk factors impacting fear intensity "come on line" in early adolescence, late adolescence, and early adulthood. As the twins aged, the influence of the shared environment declined and unique environment increased. No sex effects were found for situational fears while for animal and blood/injury fears, genetic factors in males and females were correlated but not identical. Shared environmental factors were both more important and more stable for animal fears than for situational or blood/injury fears., Conclusions: Genetic effects on fear are developmentally dynamic from middle childhood to young adulthood. As children age, familial-environmental influences on fears decline in importance.