A study was made of all live births which had been registered in New South Wales by June 30, 1970, and which occurred in the calendar years 1968 and 1969. All references in this paper relate to live births to mothers whose usual residence was within the Sydney Statistical Division (population of 2,713,000 at June 30, 1969). However, in almost all the cases, the same trends are evident in figures for the whole of New South Wales.
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J.M.S. Martins (1972) studied this question.