Unilateral Breast Enlargement (Breast Bigger on One Side)

The breasts are modified skin appendages on the chest wall. It is more prominent in post-pubertal females (after puberty) although some young boys and men may experience larger than usual breast size for various medical reasons (gynecomastia). Fat lobules within the breast largely account for its size although during pregnancy and when breastfeeding, the female breast increases further in size due to additional fat accumulation, enlargement of the milk-producing apparatus and milk stored within the breast. The size of the female breasts is determined by several factors, the most prominent of which are genetic factors and the quantity of circulating female hormones. These factors can alter the quantity of fat lobules and size of the milk-producing apparatus.

Contrary to popular belief, the two breasts are not exactly the same in size and shape. The nipples as well may also differ slightly in shape and position. However, the difference is usually so slight that it is barely noticeable. Even though most women can identify a slight difference between the breasts, it rarely poses a problem. In some instance, however, the difference may be significant enough to be clearly noticeable and cause significant psychological stress to a woman. These instances should not be confused with psychiatric conditions like body dysmorphic disorder where one has a skewed perception of their own body.

Other Signs and Symptoms

Ideally one should seek a medical opinion before diagnosing and fixating on unilateral breast enlargement. The perception of the patient is highly subjective and often influenced by several environmental factors like lighting and personal aspects. A medical professional will examine the breast more objectively, taking note of important anatomical features and ensuring that the light and position of the patient is not contributing to a distortion.

The most important aspect to consider once a unilateral breast enlargement is confirmed is to verify whether is due to the presence of lumps or tissue swelling. The latter may be due to increased tissue fluid, blood or an overall increase in the breast tissue.

The other signs and symptoms that need to be noted include :

  • Nipple distortion – inverted, everted, significant deviation from the midline compared to the other breast and other deformities as discussed under abnormal nipples.
  • Nipple discharge – clear fluid, milky discharge, foul-smelling pus, blood.
  • Redness of the skin
  • Dimpling or pitting of the skin
  • Skin rash
  • Skin discoloration like hyperpigmentation
  • Open sores (ulcers)
  • Pain or breast tenderness

Causes of Unilateral Breast Enlargement

Changes in breast size and subsequently the shape may occur with the hormonal fluctuations of the menstrual cycle and in pregnancy. Sometimes this is induced by the use of certain medication, most notably with oral contraceptives (birth control pills) and hormone replacement therapy (HRT). Although the breast enlargement in these instances is almost always bilateral (both sides), it can sometimes be unilateral (occurring only on one side). In fact, these hormone drugs are by far the most common causes of breast enlargement, whether unilateral or bilateral.

Injury to the Breast

Any trauma to the breast, whether mechanical, chemical or electromagentic, can cause an enlargement due to localized inflammation. The blood vessels may dilate and become more porous allowing serum to leak into the tissue spaces. Other features such as redness, pain and warmth may also be present. If there is a break in the blood vessel, then blood will leak out into the tissue spaces. Accumulation of blood in the tissue is known as a hematoma which can be minor (contusion) or major.


Local infections of the breast known as mastitis are common particularly in breastfeeding women. Usually only one breast is affected and swelling is therefore unilateral. The other features of inflammation are also present such as heat, redness and pain with or without a pus discharge from the nipple. Sometimes the site of the infection becomes localized and pus accumulations within this focus. In this instance it is known as a breast abscess. It may be felt as a lump in the breast.

Fibrocystic Breasts

Previously known as fibrocystic breast disease, these various changes in the breast tissue are actually not related to any disease. Instead it occurs for unknown reasons, most likely related to hormonal factors and is not associated with breast cancer. The tissue changes may include cyst formation, tissue overgrowth (hyperplasia) and scarring which come and go with time. Most women will experience these changes in the breast tissue at some point or the other in life. The breast feels lumpy, nodular or ropy and there may be pain and tenderness at time.

Breast Cysts

These are small fluid-filled sacs that develop from tissue growth that blocks the milk ducts. It is one of the tissue changes seen in fibrocystic breasts and therefore often associated with hormonal factors. These cysts are round and often described as spongy like firm grapes. Cysts may vary in size – small cysts (microcysts) may not be felt while large cysts (macrocysts) can be as large as 5 centimeters in diameter. It may also be accompanied by pain or tenderness at the site.

Benign Growths

Overgrowth of certain tissue may also give rise to lumps. These are benign tumors (non-cancerous) and need to be differentiated from malignant tumors (cancer). The two types of benign breast tumors that are commonly seen includes :

  • Lipoma – encapsulated mass of fatty tissue.
  • Fibroadenoma – masses of breast epithelium and stroma.

These breast lumps are usually painless and mobile compared to malignancies.

Breast Cancer

Cancer is an uncontrolled growth of abnormal cells. It can occur in any part of the body and the breast is a common site in females. Breast cancer has received much attention over recent years and many women are concerned that any lump or bump in the breast may be cancerous. However, only a small fraction turn out to be cancer but caution is nevertheless warranted. Self-examination is recommended for all women and regular mammograms are advisable particularly in high risk women.

Breast cancer may be asymptomatic or initially present as quite an innocuous condition. However, it is very serious and potentially life threatening. Every breast lump should therefore be considered to be cancerous until proven otherwise by diagnostic investigation. Breast cancer can cause severe symptoms such as nipple deformity, dimpling and peeling of the skin and bloody nipple discharge. Pain is not always present until very late stages. The mass is generally less well-defined than a benign growth and usually fixed.

Medication and Surgery

Apart from oral contraceptives and hormone replacement therapy, other drugs such as gonadotropin-releasing hormone and corticosteroids can also cause breast enlargement. Certain topical applications like those used for breast enlargement may cause swelling of the breast.

Any surgery to the breast will be followed by swelling due to the inflammation. This should subside within days. However, if it persists or appears to be worsening and accompanied by a fever, then it is possible that a post-operative infection or abscess is present. Scar tissue may develop around silicone implants in women having undergone breast augmentation which may account for lumps in the breast.

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