Genetics of head circumference in infancy: a longitudinal study of Japanese twins.

Objectives: Previous studies have shown strong genetic influence to head circumference (HC), but still little is known on the development of genetic etiology of HC in infancy, especially in non-Caucasian populations. Thus, we decided to analyze the genetics of HC growth in Japanese infants.

Methods: Longitudinal measures of HC were available from birth to 13 months of age in 206 monozygotic and 156 dizygotic complete twin pairs. Genetic modeling for twin data was used.

Results: We found only little evidence for sex-specific differences in the genetics of HC and thus analyzed boys and girls together. After 5 months of age the heritability of HC was high, but before that age also a substantial common environmental component was present. Not only strong genetic persistence for HC was found but also a new genetic variation emerged. New environmental variation shared by co-twins affecting HC was found until 3 months of age, and this effect was further transmitted until 1 year of age.

Conclusions: HC and its growth are strongly genetically regulated. Largely, the same genetic factors affect the variation of HC at different ages, and new genetic variation emerged during the first year of life. Knowledge on the genetic component in the variation of HC may help to design tools for defining abnormal growth of HC in population-based screenings for related disorders.