Inter-generational influences on age at onset of puberty: Hong Kong's 'Children of 1997' birth cohort

Background Early puberty often occurs in migrants from less to more economically developed locations, particularly among girls, perhaps because of mismatched inter-generational conditions. However, migrants may differ from their host population in many ways. Objective In an ethnically homogenous Chinese population in a developed environment, we examined the association of mother's growth environment (proxied by migration status) with age at onset of puberty. We assessed differences by sex and whether associations were independent of intra-uterine growth. Methods We used interval-censored survival analyses in 3832 boys and 3279 girls (follow-up rate of 92%) from the 'Children of 1997' birth cohort, comprising 88% of births in Hong Kong in April and May 1997, to examine the adjusted association of mother's migration status (born and raised in mainland China or in comparatively more developed Hong Kong), with clinically assessed age at onset of puberty (Tanner stage II for breast/genital and pubic hair development). Results Children with mothers from a less developed environment had earlier onset of breast/genital [time ratio (TR) 0.987, 95% confidence intervals (CIs) 0.980-0.993] and pubic hair (TR 0.993, 95% CI 0.986-1.000) development, independent of birth size for gestational age and socio-economic position, possibly with a more marked association in girls. Conclusion Mismatch of growth conditions between mothers and children was associated with younger age at onset of puberty. Given the association of early puberty with chronic diseases, inter-generational influences may be relevant to the emerging epidemics of these diseases in rapidly developing populations where age of puberty is declining sharply.