One Side Breast Swelling, Single Breast Enlarged, Swollen

Any breast swelling, whether it is on one side or both, is likely to be a cause for concern as the worry of breast cancer is always on a woman’s mind. While it is absolutely necessary for all breast swellings or lumps to be evaluated by a doctor, in most cases they do turn out to be non-threatening in nature and can be treated effectively.

It is important to note that if you do discover one sided breast swelling, with or without any palpable lumps, it is advisable to consult with your doctor or gynecologist first before you jump to any conclusions or decide to ignore it. The reality is that both breasts are not exactly the same size or shape, but it is usually such a subtle difference that it cannot be easily noticed.

Causes of Breast Swelling on One Side

Young girls often find that one of their breasts is slightly larger than the other. This is a normal finding and should not be a cause for concern. Women on contraceptive pills or hormone replacement therapy (HRT) may also have slight breast enlargement, usually on both sides (bilateral), but sometimes it may be only on one side (unilateral). These are two of the most common causes of noticeable one-sided breast enlargement.

Other possible causes of one-sided breast swelling may include :


Mastitis which is breast infection is common in women who are breastfeeding. It can cause swelling of the breast on one side.

A breast abscess is a collection of pus which forms within the breast as a result of infection. It usually occurs in a woman who is breastfeeding.

Cellulitis or inflammation of the skin as a result of an infection can cause swelling of the breast.


Any injury to the breast may cause a swelling due to collection of blood within the breast. A firm irregular mass due to fat necrosis may follow trauma.

Fibrocystic Breast Disease

Fibrocystic disease of the breast (fibroadenosis)  may occur in one or both breasts. It is usually seen during the child bearing years and is common in pre-menopausal women. The lumps may be painful but there is no fear of them becoming cancerous.


A fibroadenoma is also called a “breast mouse” because it is felt as a smooth swelling which is quite mobile within the breast. It usually occurs in young women, causing a painless swelling in the breast and is usually not pre-cancerous.


Cysts are little sacs filled with fluid. A cyst may cause swelling of the breast which sometimes becomes painful just before the onset of periods.

Milk cysts may form due to pockets of milk collection within the breast during lactation. These can become infected and form a breast abscess at a later stage.


Breast cancer continues to rise globally and a regular mammogram and routine breast examinations are advisable. A biopsy of a lump will assist with the final diagnosis.


A breast lipoma is a lump formed by collection of fatty tissue within the breast.

Lymphatic Obstruction

Lymphatic obstruction is the blockage in lymph flow within the breast and this may be due to a tumor, infection, injury or surgery and can cause a unilateral breast swelling.

Infants, New Born Babies

Breast lumps are sometimes seen in newborn babies and this is formed by the action of the of the maternal hormones. These usually disappear in due course, without any treatment, as the estrogen level falls.


Toughened silicone implants or scar tissue after breast augmentation surgery  may be felt as pseudolumps in the breast.

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  1. I appreciate the helpful information but I need to know more about the constantly biting, swelling and discolouration of the breast.

  2. My concern is am a 46 old male with swollen left breast which is getting worse but no pain.

  3. And when this happens within a couple of days? The breast has gotten bigger over several months, but now a bit of tenderness and pain, and the feeling like something is “there”….

  4. I have a friend who is an alcoholic, that led to kidney and liver shut down. She survived but her left breast has grown tremendously. The doctors don’t have any answers for her. She is 46 and had her menstrual cycle up until 2 months ago when she ended up in the hospital and was told that she had one week to live…any one have any answers or thoughts about the breast???

  5. This is for my g.f. she is just 20 years, but she has history of alcohol, smoke, she is also pcos patient. She is feeling painless one side breast enlargement. She gained some weight during in last month, please suggest something.

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