PREVALENCE OF NEURAL-TUBE DEFECTS IN SOUTH-AUSTRALIA, 1966-91 - EFFECTIVENESS AND IMPACT OF PRENATAL-DIAGNOSIS

Objective-To determine trends in total prevalence of neural tube defec ts in South Australia during 1966-91, the impact of prenatal diagnosis on birth prevalence, and the effectiveness of prenatal screening for neural tube defects in 1986-91. Design-All births and terminations of pregnancy affected by neural tube defects and information on prenatal screening were ascertained from multiple sources including the South A ustralian perinatal and abortion statistics collections, birth defects register, and state maternal serum a fetoprotein screening programme. Setting-Southern Australia. Subjects-All 1058 births and terminations of pregnancy affected by neural tube defects in 1966-91. Main outcome measures-Total prevalence and birth prevalence of individual and all neural tube defects. The proportion of screened cases detected prenata lly.Results-Total prevalence of neural tube defects during 1966-91 was 2.01/1000 births with no upward or downward trend. However, birth pre valence fell significantly (by 5.1% a year), with an 84% reduction fro m 2.29/1000 births in 1966 to 0.35/1000 in 1991 (relative risk=0.16, 9 5% confidence interval 0.07 to 0.34). The fall was 96% for anencephaly and 82% for spina bifida. 85% of defects, both open and closed, were detected before 28 weeks' gestation in women screened by serum alpha f etoprotein or mid-trimester ultrasonography, or both, in 1986-91 (99.0 % for anencephaly and 75.7% for spina bifida). Conclusions-While the t otal prevalence of neural tube defects in South Australia remained sta ble, prenatal diagnosis and termination of pregnancy resulted in an 84 % fall in birth prevalence during 1966-91. Screening detected over fou r fifths of cases in 1986-91.