Improvements in the technological ability to diagnose birth defects early in pregnancy have focused attention on ways to correct certain fetal defects by manipulating the mother's diet. For example, researchers are investigating the use of vitamin- mineral supplements to prevent neural tube defects--that is, failure of the fetus's neural tube to close because of spinal cord abnormalities. Other investigators are researching ways maternal nutrition can help fetuses with inherited birth defects, usually inborn errors of metabolism, in which certain nutrients are not processed normally.