INTERNATIONAL STUDY OF SEX-RATIO AND TWINNING OF NEURAL-TUBE DEFECTS

The goal of this study, based on information collected from eight cong enital malformation registry programs around the world, was to analyze sex and twinning of affected infants and fetuses, according to type o f neural tube defect (NTD) and other variables. The type of defect and its location, whether it occurred in isolated form or was associated with other malformations, population from which data were retrieved, v ital status of the infant, and gestational age of the fetus/infant wer e considered. The material included 3,416 infants and 168 selectively aborted fetuses with anencephaly, 4,830 infants and 76 fetuses with sp ina bifida, and 1,022 infants and 19 fetuses with encephalocele. Less than 20% of infants with anencephaly or spina bifida and more than one -third of infants with encephalocele had associated malformations. A f emale excess was found among all infants with NTD but a male excess in fetuses delivered spontaneously before week 20, indicating selective male late fetal deaths. Sex ratio varied by type of NTD, vital status at birth, presence of associated malformations, and year of birth, wit h an increasing sex ratio for anencephaly during the 1960s and 1970s. Twinning was positively associated with NTD but the extent of this ass ociation varied with NTD type and program, while the distribution by s ex type of pair of twin appeared to be similar to that of all births. Twins concordant for anencephaly or encephalocele were mainly found wh en the defect occurred as part of a syndrome, and only in like-sexed p airs. Twins concordant for spina bifida had the isolated form of the d efect and 5 of 6 pairs were like-sexed. Thoracic spina bifida has a se x ratio more like anencephaly than lumbosacral spina bifida has, but t winning rate is more similar in anencephaly and lumbosacral than in th oracic spina bifida. (C) 1994 Wiley-Liss, Inc.