[Abnormalities of the neural tube in twins].

We have studied neural tube malformations in twins in order to research into the role of genetic and environmental factors. 12 pairs of twins in which one child had a neural tube defect were studied in Brittany, which is a Celtic country. We found no evidential agreement about the role each factor played. On the other hand there was an excess of twins in the siblings of those with neural tube defects, especially in the siblings of the mothers. There were more dizygotic twin mothers. Analysing the literature has made it possible for us to find a level of agreement of 7.5% for monozygotic twins and 4.6% for dizygotic twins. This last figure corresponds to the recurrence rate found after one case. The aetiological theories are reviewed. Among factors bringing about neural tube defects would seem to be the microenvironment of the uterus and the delay between ovulation and fertilization and implantation of the fertilized egg. Nutrition of the embryo and possible vitamin deficiencies could explain this inter-action between the mother and the fetus. If there is a genetic factor, it is more likely to be maternal than fetal.