During pregnancy, the breasts undergo significant hormonal changes primarily due to elevated levels of estrogen, progesterone, and prolactin. These hormones stimulate glandular hypertrophy and hyperplasia, leading to an increase in breast size. Additionally, increased blood flow and fluid retention cause breast tenderness. These changes prepare the breast tissue for lactation. Hence, the most prominent clinical change observed is increased size and tenderness of the breasts.

Reference: Gray's Anatomy, Standring S., 42nd Edition.