Shift of Childbirth to Younger Mothers, and Its Effect on the Incidence of Mongolism in Victoria, Australia, 1939–1964

When a comparison was made between Victoria, Australia and England and Wales, it was found that between 1939-64 there was a great similarity between percentage distribution of births by quinquennial age groups, a common trend to a greater proportion of births to younger mothers, and a decreasing mean maternal age at birth. Expected percentage incidences were calculated, and for each country there was a gradual fall from 1939-64 -- 17% for Victoria and 22% for England and Wales. The estimated number of mongols born in Victoria for any given year was obtained from the annual number of births and the corresponding expected incidence, the number varying jointly with both variables. In 1964 the number of live births had risen to 64,990, but the expected incidence had dropped to 0.131. As the number of mongols born varies jointly with the number of live births and with expected incidence, mongol births in 1964, as compared with those in 1939, will be 48 x 64,990/30,493 x131/158, which is 85, the relatively larger ratio of live births outweighing the effect of the smaller ratio of incidences. The net result of the 2 factors operating together resulted in an increase of mongol births from 48 to 85 over this period. From 1964-67 there was a small fall in the number of births and another fall in incidence. A similar calculation shows that the estimated number of mongols born was reduced from 85 in 1964 to 80 in 1967. As for the future, the annual number of births in Victoria now seems to be rising substantially and even if the trend to younger mothers continues, it apears that the increase in the population of births will result in an estimated increase of mongol births. If either of both of these rates change, this estimate would have to be reassessed. observed mean percentage incidences of mongol births in Victoria for 1942-57, for the 3 5-year cycles, were in agreement with the corresponding decreasing rates which were to be expected in accordance with diminished maternal age.