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When girls hit puberty, estrogen is released into the body and the breasts begin to grow. This tends to coincide with the beginning of menstruation. As the breasts grow, the milk ducts in them increase in size, causing the breasts to expand at the nipple. Then the breast tissue itself expands. During pregnancy, the hormone progesterone readies the breasts to produce milk. The breasts grow larger due to additional fat and swollen milk glands.

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10y ago

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