we conducted a study of published work to evaluate the evidence for the hypothesis that environmental exposure and parental occupation are risk factors for neural tube defects . 
 as other risk factors such as maternal illnesses , medication , and dietary factors have been reviewed before , this review only summarizes this information . in studies concerning environmental pollution , 
 only a few weak associations were found . 
 it appears that specific studies on the topic of parental occupation and neural tube defects are scarce . 
 therefore , studies on broader malformation categories , such as central nervous system defects , were also taken into account . 
 both maternal and paternal occupation seem to be associated with the occurrence of neural tube defects . 
 however , results are not always consistent with each other , and relevant recommendations concerning prevention thus can not be given before more studies with larger populations are conducted to confirm or refute the findings reviewed.imagesp140-a