It's a natural process in the female body that, starting from ovulation, with the increase in progesterone levels, the breast's glandular tissue begins to grow, the cells of the gland lobules start to divide so that by the time the baby is born (in case of fertilization), they are prepared to feed the offspring.

Thus, it's a well-known fact that in case of demand (pregnancy), the mammary gland tissues begin to proliferate so that the "milk factory" can meet the expectations by the due date - because the amount of milk produced depends on the number of mammary glands.