Fertility, fecundity and twinning: a comparison of the trends in births and twinning in Australia from 1854 to 1982.

The trends in births and in twinning in Australia are compared from 1854 to 1982. Until about 1930 the twinning rate increased as births fell. The two rates then became concordant. The relationship of fecundity to twinning is discussed. Comparison with other countries is made. The period embraces the demographic transition when changing contraceptive practices would have had a variable effect on twinning.