Final answer:
Folic acid is the necessary nutrient for pregnant women to prevent neural tube defects like spina bifida and anencephaly.
Step-by-step explanation:
The necessary nutrient for pregnant women to prevent neural tube defects is folic acid, which is a B vitamin. Folic acid is essential for the healthy development of the neural tube in the early stages of pregnancy. A deficiency of folic acid can increase the risk of neural tube defects such as spina bifida and anencephaly.