Final answer:
The highest fetal demand for iron occurs in the third trimester, which is a period of rapid growth and significant development, including the onset of erythrocyte synthesis by the bone marrow and increased nutrient demands for preparing the fetus's bodily functions outside the womb.
Step-by-step explanation:
Fetal demand for iron is highest during the third trimester of pregnancy. This is the period characterized by the greatest growth of the fetus, where rapid weight gain occurs and the bone marrow takes over erythrocyte synthesis. It's also the stage where the fetus's lungs begin producing surfactant, an important substance for lung development and function after birth.
During the third trimester, there is a heightened need for various nutrients to support the substantial development of all organs and systems. Increased fetal demand for nutrients, including iron, is critical as the fetus prepares for the transition to a stable body temperature outside the womb. The third trimester culminates in labor and delivery, making the provision of adequate nutrients essential.