Which deficiency in the mother has been primarily associated with neural tube defects in the fetus?

Folic acid deficiency in the mother has been primarily associated with neural tube defects in the fetus. Folic acid is crucial for the development of the neural tube in the early stages of pregnancy. Its deficiency can lead to neural tube defects, such as spina bifida or anencephaly. Iron deficiency is not directly related to neural tube defects but can cause other complications in pregnancy. Vitamin B12 is important for neurological function but is not the primary cause of neural tube defects. Vitamin E deficiency is not associated with neural tube defects in the fetus.