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How do estrogen and progesterone affect the breasts during pregnancy?

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Close-up of a home pregnancy test device showing a blue plus sign in the result window, indicating a positive result for pregnancy. — pregnancy Devices for individual use at home detect pregnancy by measuring levels of the hormone human chorionic gonadotropin (hCG) in urine.

During pregnancy, significantly increased levels of estrogen and progesterone drive substantial physiological changes in the breasts to prepare them for future lactation.1

These hormones cause the breasts to become enlarged, sensitive, and tense. As pregnancy progresses, the areola darkens in color, and the veins beneath the skin become more prominent. Additionally, the sebaceous glands around the nipple, known as the glands of Montgomery, become more noticeable.12

Functionally, estrogen and progesterone prepare the breast tissue for the action of prolactin, the hormone responsible for milk production. While progesterone specifically stimulates the development of the glands responsible for producing milk, high levels of these ovarian hormones during pregnancy are thought to inhibit the actual secretion of milk until after delivery. Once the pregnancy concludes and hormone levels decrease, the inhibition is lifted, allowing prolactin to induce the secretion of milk. During the later stages of pregnancy, a milky fluid called colostrum may also be expressed from the ducts.13