Cases of spina bifida cystica, encephalocele, and anencephaly occurring over a 9-year period, 1965 to 1973, in New South Wales, Australia, were identified. A low frequency of 1·1 for spina bifida and encephalocele (SB) and 0·9 for anencephaly (A) was found.
Secular trends parallel to those observed in the northern hemisphere were noted.
Detailed analysis of 1575 cases showed an excess of births in spring, corresponding with conception in the summer months, after correction for shorter gestation in anencephalus, which varies from the peak of spring conceptions observed in British studies.
An excess of female cases for each abnormality and a social class effect with a deficit of cases in classes I and II and an excess in classes IV and V and ex-nuptial births were apparent. The first birth rank for younger mothers did not show a significantly increased risk; however, the effect of high birth rank and older maternal age was more significant. Migration studies showed that in migrating from areas of high incidence these parents maintain a higher risk than the Australian population.
The highest risk group was that in which both parents were born in the UK, and the next highest that in which an English-born mother was married to an Australian father.
Mothers from Malta, and either or both parents from Lebanon, Egypt, and Austria were also at high risk.
Part-aboriginal children had a higher risk rate for ASB than white Australian children.
The proportion of older sibs affected was 4·12% of sibs of both sexes of an index case of spina bifida, and 3·19% of an index case of anencephaly. The abnormalities alternate or recur in families. An increased perinatal mortality rate in sibs was shown.
Twin studies showed a higher concordance rate for monochorionic pairs.
A sequential interaction in an excess of opposite sex sib before an index case was apparent.
The results of this study support a multifactorial aetiology for ASB resulting from genetic environmental interaction.
New South Wales is the eastern coastal state of Australia, with an area of 309 433 square miles and a population of 4 640 800 at the 1971 Census. The continent is geographically isolated with a large migrant population, seasonal reversal, and a hot climate.