Genetic and environmental factors affecting birth size variation: A pooled individual-based analysis of secular trends and global geographical differences using 26 twin cohorts

Background: The genetic architecture of birth size may differ geographically and over time. We examined differences in the genetic and environmental contributions to birth weight, length, and ponderal index (PI) across geographic-cultural regions (Europe, North-America and Australia, and East-Asia) and across birth cohorts and how gestational age modifies these effects.

Methods: Data from 26 twin cohorts in 16 countries including 57613 monozygotic and dizygotic twin pairs were pooled. Genetic and environmental variations of birth size were estimated using genetic structural equation modeling.

Results: The variance of birth weight and length was predominantly explained by shared environmental factors, whereas the variance of PI was explained both by shared and unique environmental factors. Genetic variance contributing to birth size was small. Adjusting for gestational age decreased the proportions of shared environmental variance and increased the propositions of unique environmental variance. Genetic variance was similar in the geographic-cultural regions, but shared environmental variance was smaller in East-Asia than in Europe and North-America and Australia. The total variance and shared environmental variance of birth length and PI were greater from the birth cohort 1990-1999 onwards compared with the birth cohorts from 1970-1979 to 1980-1989.

Conclusion: The contribution of genetic factors to birth size is smaller than that of shared environmental factors, which is partly explained by gestational age. Shared environmental variances of birth length and PI were greater in the latest birth cohorts and differed also across geographic-cultural regions. Shared environmental factors are important when explaining differences in the variation of birth size globally and over time.