Secular trends in anencephalus prevalence in New Zealand

Although the aetiology of anencephalus is unknown, changes in prevalence over time are thought to provide strong evidence of the effect of environmental factors in causation. In a number of countries the rate of this neural tube defect is declining. The rate in New Zealand has also fallen significantly (p less than 0.001) from 1.17/1000 total births in 1968 to 0.39/1000 in 1983, with the greatest decrease occurring in the female rate (approximately 4.16% per year). The aetiological implications of these findings and their consequences for programmes of prenatal diagnostic testing and maternal screening are discussed.