ABSTRACT
Neural tube defects (NTDs) remain among the most common types of congenital anomalies, although prevalence rates at birth of these defects have been declining in certain areas of moderate or high risk. A number of features — long term trends, seasonal variation, and social class distribution — of the descriptive epidemiology of NTDs have been regarded as consistent with a dietary etiology. The relationship between valproic acid and NTDs may be one of the strongest specific associations yet identified in the epidemiology of NTDs. Potentially the most clinically important exposure to exogenous sex hormones is inadvertent use of oral contraceptives during early pregnancy or where conception occurs before the drug has been fully cleared from the maternal tissues. No association between oral contraceptives and NTDs were found in three cohort studies, but in each of these, the number of cases of NTDs was small.
