What happens to parathyroid hormone during pregnancy?

• A. Increases in the first trimester, then decreases
• B. Decreases in the first trimester, then increases to term
• C. Remains unchanged
• D. Increases steadily throughout pregnancy

Answer: (B)

The body adapts calcium balance to support fetal bone development. Early in pregnancy, the placenta and kidneys increase production of calcitriol, which boosts intestinal calcium absorption. This higher absorption lowers the need for PTH, so parathyroid hormone levels fall in the first trimester. As the pregnancy advances, fetal calcium demands rise for skeletal mineralization, and maternal calcium management shifts toward more PTH-mediated mobilization to maintain serum calcium, so PTH increases toward term. This pattern—a drop in the first trimester followed by an rise to term—best fits the physiological changes in calcium homeostasis during pregnancy.