What physiological change increases maternal calcium absorption during pregnancy?

The physiological change that increases maternal calcium absorption during pregnancy is primarily related to hormonal adjustments. During pregnancy, there is an increase in the levels of hormones such as calcitriol (the active form of vitamin D) and parathyroid hormone, which enhance the absorption of calcium from the gastrointestinal tract. This is crucial because the needs for calcium increase significantly during pregnancy due to the developing fetal skeleton and the mother's expanding physiological demands for calcium.

While it is true that metabolic demand increases during pregnancy, this option does not specifically highlight the mechanism through which calcium absorption is enhanced. The correct answer emphasizes the role of hormonal adjustments, which directly facilitate more efficient absorption of calcium, ensuring that both the mother and fetus have adequate calcium for health and development. Moreover, while fetal demand for energy and nutrients generally increases, it is the hormonal changes that are directly responsible for the enhanced calcium absorption. Thus, focusing on the hormonal changes offers a clear explanation of the physiological mechanism underlying the process.