Environmental factors in the etiology of neural tube defects: a negative study.

A case-control study was made on women who had an infant with a neural tube defect and twice as many controls. Cases and controls were selected from a national medical birth registry from which prospectively collected data on previous pregnancy, contraceptive use, and smoking were also retrieved. Women's occupation was either retrieved from that registry or from census information which was partially checked with direct interview. Information on environmental characteristics of the place of living of the women in early pregnancy was given by field workers who were unaware of the status of case or control. Cases and controls showed no statistically significant difference with regard to previous pregnancies and use of contraceptives at or the month before the time of conception. Case women smoked 10 cigarettes or more a day slightly more often than control women did but the difference was not statistically significant. No statistically significant difference in occupation distribution was seen between cases and controls (but cases had slightly more occupations where chemical exposure is likely), and no differences in home environment could be found. It is concluded that the factors studied play no major etiological role for the origin of neural tube defects in Sweden.