Prescription data for the three months before the last menstrual period and for the first trimester of pregnancy were obtained for 764 mothers whose children had a defect of the central nervous system and for an equal number of mothers of control babies born from the same doctors' practices. There was a statistically significant difference overall between the numbers of mothers who were prescribed drugs in the study and control groups during the trimester before the last menstrual period but no such difference was found for the first pregnancy trimester, nor was there a significant difference for any specific group of drugs. For a composite group of non-steroid anti-inflammatory drugs, salicylates, and sulphasalazine there was a significant difference for the trimester before the last menstrual period. There are arguments against such an artificial grouping, however, and when the individual drugs were considered the comparisons were no longer significant. The odds ratios for all medicines containing folic acid taken in the trimester before the last menstrual period were considerably less than unity, in contrast with nearly all other comparisons. This supports a suggested protective effect against neural tube defects of folic acid supplements begun before the onset of pregnancy but the odds ratios of these comparisons were not statistically significant.
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