NEW YORK — For decades, pregnant women and women who may become pregnant have been advised to take folic acid to help prevent certain birth defects. But a new study suggests it may be possible to get too much of a good thing — very high levels of the vitamin in mothers' blood at the time of childbirth was linked to higher risk of their children developingautism years later. Other research points to an opposite relationship between folic acid and autism, showing that adequate amounts of the vitamin at the time of conception can significantly reduce the risk.