Secular trends of neural tube defects by demographic subgroups in Norway, 1967-81.

The secular trends of NTDs by subgroups were investigated using data from the population-based Medical Birth Registry of Norway. During 1967-81, rates of total congenital malformations significantly increased by 4.1% annually. In contrast, there was an average annual decline of 1.6% in anencephaly rates and 0.4% in spina bifida rates. Annual rates of NTDs among twins decreased more (3.8%) than among singletons (1.6%). Females had an annual decline in rates (2.2%) that was greater than in males (0.7%). Separation of NTDs into two sub-groups based on the presence of multiple major malformations, revealed a significant decline of 2.1% annually in singles, whereas rates in multiples showed an increase of 3.3% annually. The different prevalence patterns revealed in various subgroups strongly suggest different etiologic entities. These should be further refined for elucidating etiologic factors. Furthermore, in prediction of current cases and evaluation of the effects of health services, the proportion of the various subgroups present should be considered.