Trends in neural tube defects 1980-1989

Abstract

Objectives: To determine trends in prevalence of neural tube defects in Australia over the past decade; to investigate the impact of terminations of pregnancy on the birth prevalence of neural tube defects; and to provide a baseline against which to evaluate the potential effects of preventive public health measures.

Data sources: Two population-based registries in Victoria and Western Australia and a statewide survey from Tasmania.

Design and method: A cohort study. Data on the numbers and the prevalence proportions of neural tube defects from three States--Western Australia, Victoria and Tasmania--were compared by Poisson regression.

Results: The prevalence proportions for all neural tube defects for all three States remained level over the study period 1980-1989. Terminations of pregnancy for all neural tube defects increased significantly over the decade in all three States. By 1989, 39.9% of all neural tube defects were ascertained as terminations before 20 weeks' gestation, while 10 years previously only 2.9% were. The percentage of cases of anencephaly ascertained as terminations of pregnancy increased from 4.8% in 1980 to 58.6% in 1989. Corresponding figures for spina bifida were 1.4% (1980) and 26.9% (1989), and for encephalocele 0% (1980) and 12.5% (1989). This increase in terminations was associated with a decrease per year in the birth prevalence proportion for anencephaly of 7.0%, for spina bifida of 4%, and for encephalocele of 11.0%, and a reduction in the risk of a birth with a neural tube defect in 1989 of 47% compared with the risk in 1980.

Conclusion: Complete ascertainment of all terminations for neural tube defects as well as births with neural tube defects is necessary to provide reliable baseline data on the prevalence of neural tube defects. Such data are essential in evaluating primary preventive measures such as the effect of an increase in folic acid intake by women of child-bearing age.

PIP: The purpose of this study was to document trends in the prevalence of neural tube defects (anencephaly, spina bifida, and encephalocele) between 1980 and 1989 in Tasmania, Victoria, and Western Australia. The results were considered useful in providing a baseline for evaluation of potential public health measures, such as universal administration of folic acid. Births included those cases of prenatal diagnosis of neural tube defect where labor was induced after 20 weeks gestation, but before term. Data were obtained from provincial registries and surveys, which are specified. Prevalence was established as the proportion of live births and stillbirths with neural tube defect and pregnancy terminations with neural tube defect before 20 weeks duration, divided by live births and stillbirths. Outcomes could be termination, live or stillbirth, and the total of both terminations and live and stillbirths. Poison regression analysis was performed for all 3 Australian states and for each of the 3 neural tube defect types and total. The results showed stable trends over the decade for all states. Significant differences were found in the prevalence of defects; there was a greater risk of defects in Western Australia compared with Victoria or Tasmania. The prevalence of pregnancy terminations before 20 weeks due to defects increased significantly in Victoria and Western Australia, e.g. terminations in 1980 were 2.9% and 39.9% in 1989. Anencephaly terminations for all 3 states increased from 4.8% in 1980 to 58.6% in 1989. The risk of births with a neural tube defect was reduced by half by 1989. THere was a significant decline in births with anencephaly for all 3 states. Anencephaly data showed Victoria and Tasmania with significantly lower levels than Western Australia. Terminations for anencephaly were significantly lower in Victoria than in Western Australia. Western Australia saw a significant increase in anencephaly terminations over the decade. Spina bifida birth prevalence declined by 4%/year for all 3 states and the terminations for spina bifida increased by 17%/year. Terminations were 1.4% in 1980 and 26.9% in 1989. Birth prevalence for encephalocele declined by 11% during the decade. The prevalence proportion of neural tube defects in 1990 was 1.47/1000 births. Prevalence declined between 1975 and 1981 and remained stable thereafter. In all 3 states, 40% of all neural tube defects were screened before 20 weeks.