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Vitamins During Pregnancy and Neural Tube Defects

Samuel Shapiro, MB; Allen A. Mitchell, MD
JAMA. 1990;263(20):2748. doi:10.1001/jama.1990.03440200049014.
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To the Editor.—  In their discussion of the apparent protective effect of multivitamin/folic acid supplements on the risk of neural tube defects (NTDs), Milunsky et al1 claim that "the timing of the interviews in early pregnancy and the fact that the vast majority of mothers were unaware of pregnancy outcome minimized the possibility that this finding was due to their selective participation or biased recall of supplementation."With regard to selective participation, all women were recruited from those whose α-fetoprotein or amniocentesis samples were submitted to the authors or their colleagues. However, unless all samples (or a random sample) from each participating center were included, the possibility of selection bias cannot be excluded. Such a bias becomes a strong possibility for the 19% of amniocentesis samples analyzed elsewhere, since those women may have been referred preferentially when their samples yielded a diagnosis of NTD and there was no history


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