The secular variation of Australian twin births over fifty years.

Twins born during the fifty years 1920-69 in Australia were analysed statistically and their sex composition was found to be homogeneously distributed in a geographical but not a secular manner. The twinning rate reached a maximum in 1953 after which it fell to pre-war levels. This behaviour was due to dizygotic twins, and the role of intermediate maternal age at parturition as a contributing factor is discussed. Considered in relation to all twin maternities, however, the dizygotic rate declined significantly and linearly with time. This could be attributed to the gradual secular decline in maternal age.