Seasonal variations of births in Australia from 1911 to 1940 and 1962 to 1979 are analysed using stepwise periodic regression analysis. It is shown that the seasonality pattern has changed significantly from a September peak in the early 1960s to a February-March peak in the late 1970s. There also appears to be a significant geographical trend in seasonality of births with a February-March peak in the more northern States of Australia giving way to a September-October peak in the southernmost States. The seasonality of Australian births in 1976 to 1979 is shown to be independent of legitimacy and birth order but dependent on maternal age. The evidence suggests that environmental factors play a more important role than sociocultural factors in the causation of seasonal variations in births.