| Summary: | Several investigators have suggested that "upper" neural tube defects (NTD's)--anencephaly, encephalocele, and thoracic spina bifida--are etiologically different from "lower" NTD's (lumbo-sacral spina bifida). This hypothesis was primarily based on the observations that the two types have different sex ratios and recurrence rates and that the NTD cases within one sibship are concordant for NTD type. In order to test this, the above figures were calculated in a sample of NTD probands from Montreal and Newfoundland. The findings were not consistent with the hypothesis. However, a previously unreported finding was observed: the frequency of twinning was significantly higher in the near relatives of upper NTD probands than in those of lower NTD probands or of controls. This curious association between upper NTD's and twinning may be explained by a familial factor predisposing to a delay early in development. This delay could also explain any differences observed in upper and lower NTD groups.
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