Which nutrient deficiency may result in decreased birth weight due to low bone density in a fetus?

A deficiency in calcium during pregnancy can lead to decreased birth weight due to its critical role in fetal bone development. Calcium is vital for the growth and mineralization of bones, and an adequate supply is necessary to ensure that the fetus can develop a strong skeletal structure. Insufficient calcium can result in low bone density, which may negatively impact the overall growth and weight of the fetus at birth.

While other nutrients are important in pregnancy, they play different roles. For example, iron is crucial for preventing anemia and ensuring adequate oxygen supply to the fetus, while zinc supports immune function and cellular growth. Folic acid is essential for neural tube development and preventing defects, but it does not primarily influence bone density. Therefore, a lack of calcium directly impacts fetal bone growth and weight, making it the correct answer in this context.