**Core Concept**
During pregnancy, the demand for certain nutrients, including trace elements, increases to support fetal growth and maternal health. Some trace elements have limited dietary sources or are not efficiently absorbed from food, making supplementation crucial.

**Why the Correct Answer is Right**
Iron is the primary trace element that cannot be completely supplemented by diet in pregnancy. This is because the recommended daily intake of iron is higher during pregnancy, and dietary sources may not be sufficient to meet this increased demand. Iron deficiency and anemia are common complications in pregnancy, emphasizing the need for supplementation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Zinc can be adequately supplemented through a balanced diet rich in zinc-rich foods like oysters, beef, and chicken. While zinc deficiency can occur in pregnancy, it is not as prevalent as iron deficiency.
* **Option B:** Iodine is a critical trace element for thyroid function, and its deficiency can lead to cretinism in the fetus. Iodine supplementation is essential, especially in areas with iodine-deficient soil and water. However, dietary sources can provide sufficient iodine if the diet is well-planned.
* **Option C:** Calcium is essential for fetal bone development, and dietary sources like dairy products, leafy greens, and fortified plant-based milk can meet the increased demand during pregnancy. However, calcium supplements may be necessary for women with restricted diets or those with a high risk of osteoporosis.

**Clinical Pearl / High-Yield Fact**
Adequate iron supplementation during pregnancy is crucial to prevent iron deficiency anemia, which can lead to preterm labor, low birth weight, and increased risk of maternal and perinatal mortality.

**Correct Answer:** B. Iodine.