Why do the RDAs for calcium and vitamin D generally remain unchanged during pregnancy despite increased fetal needs for calcium?

Pregnancy increases maternal absorption efficiency and alters physiology so that more dietary calcium is absorbed, and vitamin D's role in absorption means its RDA also remains unchanged

Because the fetus obtains calcium exclusively from maternal bone breakdown and not from dietary sources, so dietary RDAs are irrelevant

Because pregnant women are advised to avoid dairy and sun exposure, which balances the demand and supply without changing RDAs

Because the placenta produces large amounts of calcium and vitamin D, eliminating the need for increased maternal intake