Progesterone is a female fertility hormone which can cause breast swelling and tenderness. Most studies show progesterone therapies do not increase the risk of breast cancer.

Progesterone is one of the most important female fertility hormones. This hormone, in conjunction with the another female hormone, estrogen, are believed to be responsible for many of the changes women feel in their breasts throughout their lives, according to Johns Hopkins 1.

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Progesterone and The Menstrual Cycle

Progesterone is secreted by a temporary gland (the corpus luteum) that forms after ovulation during the second half of the menstrual cycle, according to Hormone.org 2. Progesterone prepares the lining of the uterus (endometrium) for the implantation of a fertilized egg. Progesterone also prompts the body to create milk glands in the breasts, per Johns Hopkins. This can cause swelling in the breasts as well as soreness and pain.

If a woman doesn't become pregnant, the corpus luteum breaks down and progesterone levels decrease. As a result, the uterus releases the endometrium, causing menstruation, per Hormone.org. Breast will also reduce in size.

Progesterone and Fertility

If a woman does become pregnant, the corpus luteum will continue to produce progesterone for about another 10 weeks, according to the University of California, San Francisco (UCSF) 35. This stimulates further breast growth, per Johns Hopkins. As during the second half of the menstrual cycle, progesterone causes the formation of milk glands during pregnancy. This can cause breast soreness, swelling, and nipple tingling. After about 10 weeks, the placenta takes over the production of progesterone.

As described, progesterone is critical for successful conception. So much so that the National Infertility Association refers to progesterone as "the pregnancy hormone." But, for a variety of reasons, some women can struggle to conceive 4. Assisted Reproductive Technology such in-vitro fertilization (IVF) often requires progesterone hormone supplementation in the form of vaginal gels, inserts or injections 25. These can have a number of side effects including breast pain, swelling and tenderness. Other common side effects include headaches, mood changes, stomach discomfort and cramps, according to the University of Michigan 5.

Read more: Tips on Achieving Pregnancy at Age 38

Progesterone and Birth Control

A form of progesterone-- progestin-- can also be used for birth control 26. There are two types of progestin birth control pills: A combination of estrogen and progestin and progestin-only, according to the American College of Obstetricians and Gynecologists 6. Progestin-only pills are sometimes referred to as mini-pills.

Both estrogen/progestin and progestin-only pills can cause breast tenderness or swelling, according to Drugs.com. Other common side effects include fluid retention or weight gain, nausea, spotting between periods, and mood changes.

If you're worried about the side effects of birth control pills, discuss your concerns with your doctor. There are many contraceptive options, including birth control skin patches, injections, or vaginal rings.

Read more: The Side Effects of Stopping Birth Control

Progesterone After Menopause

HRT may carry some risks. According to MedlinePlus.gov, these include:

  • an increased chance of heart attack
  • stroke
  • blood clots,
  • breast cancer

However, there is controversy as to whether or not HRT is actually a risk factor for these conditions. The Cleveland Clinic notes that women who start HRT at in their 50's are less likely to be at risk conditions like heart disease versus women who start HRT in their 60's 7. Additionally, the Cleveland Clinic cites studies which found that HRT does not increase the risk of heart disease or breast cancer 7.

Is There A Progesterone-Cancer Link?

Roshni Rao, MD, Chief of the Breast Surgery Program at Columbia University Medical Center, tells LIVESTRONG.com that, "Most of the studies show that progesterone does not increase the risk of breast cancer 5. [However], there is a very small, increased risk with birth control pills, especially combination birth control [progestin and estrogen]," says Rao. But the risk is so small, Rao considers progesterone-related treatments to be safe. She says studies of progesterone on its own-- that is, without estrogen-- have not found a breast cancer risk. "The risk of being overweight and breast cancer is much, much, higher than birth control and breast cancer," says Rao.

Read more: 5 Things You Need to Know About Breast Cancer Symptoms

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