The Effects of Progesterone on the Breasts

Progesterone is a steroid hormone made primarily in the ovaries but also by the placenta during pregnancy. Progesterone acts with other female hormones, such as estrogen, to maintain female reproductive health and female sex characteristics. It has several effects on breast tissue during puberty, menstruation and pregnancy. According to a review in the October 2008 issue of "Steroids," progesterone also influences the growth of cancerous tissue in the breast, although its effects on breast cancer cell growth are controversial.

Stimulates Growth and Development

Estrogen is the primary hormone that stimulates breast growth during puberty, but progesterone is required to convert the female breast into a milk-producing organ. Dr. Carol Lange reports in the article in Steroids that progesterone, along with other hormones, stimulates development of the mammary glands in the breast.

Increases Fluid Retention During Menstrual Cycle

Soon after ovulation, progesterone levels begin to rise and peak during menses. Progesterone increases fluid retention during the latter part of the menstrual cycle. The Ohio State University Medical Center says the fluctuations in progesterone that occur during the menstrual cycle contribute to the cause of the characteristic breast tenderness that occurs just prior and during menstruation.

Helps Prepare the Breast for Lactation

If a woman becomes pregnant, progesterone levels increase substantially. One of the functions of progesterone during pregnancy is to promote further development of the glandular tissue in the breast so that it will secrete milk. Progesterone, estrogen, prolactin and other hormones interact in a complex manner during pregnancy to prepare the breast for lactation. Progesterone stimulates gland and duct development and the expression of milk proteins but does not stimulate milk release. In fact, an article in a 1997 issue of the journal Endocrine Reviews notes that progesterone blocks milk production until delivery.

Possible Increased Risk of Breast Cancer

The role of progesterone in breast cancer is highly controversial. The results from many animal and laboratory studies indicate progesterone increases and inhibits breast cancer cell growth; the effects are highly dependent on the actions of other hormones. Results from the Million Women Study published in the August 2003 issue of Lancet suggest that greater than 10 years of using progesterone and estrogen combination hormone replacement therapy significantly increases the risk of breast cancer in postmenopausal women. From this study, the authors estimate five additional breast cancers per 1,000 users of estrogen-only preparations and 19 additional breast cancers per 1,000 users of estrogen-progesterone combination therapy.

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