A lump in the armpit during pregnancy can cause anxiety. This concern is understandable, given the location near breast tissue and lymph nodes, but the vast majority of these masses are benign and related to the normal physiological changes of pregnancy. The body undergoes hormonal transformations in preparation for childbirth and lactation, which often makes pre-existing, unnoticed structures more prominent. While most armpit lumps are harmless, any pregnant individual who discovers a new mass must report it to their healthcare provider for a professional evaluation.
Hormonal Changes and Accessory Breast Tissue
The most common reason for a lump in the armpit during pregnancy is the activation of accessory or ectopic breast tissue. This tissue is a remnant from embryonic development along the “milk line” (or mammary ridge), which extends from the armpit down to the groin. Accessory breast tissue, medically known as polymastia, is present in a small percentage of the population but often remains inconspicuous until hormonal shifts occur.
The surge of hormones, particularly estrogen and progesterone, during pregnancy stimulates the growth and development of breast glandular tissue. This includes the main breasts and any accessory tissue located outside the normal boundaries, most frequently in the armpit (axilla). A specific extension of breast tissue that naturally reaches into the armpit is called the Tail of Spence, and this area commonly swells. Since this accessory tissue contains milk ducts and glandular components, it reacts to pregnancy hormones.
The tissue often feels like a soft, sometimes tender mass under the skin. This type of lump is frequently bilateral, and its size can fluctuate as the pregnancy progresses and the body prepares for milk production. The swelling, known as axillary hypertrophy, is a natural response to the hormonal environment and typically decreases after delivery when hormone levels drop or lactation stabilizes.
Swollen Lymph Nodes and Immune Response
Another frequent cause of armpit lumps is the enlargement of the axillary lymph nodes, which are part of the body’s lymphatic and immune systems. Lymph nodes are small, bean-shaped structures that filter lymph fluid and trap foreign particles, bacteria, and infection-fighting white blood cells. Swelling in these nodes indicates that the immune system is actively responding to an irritant or infection.
While pregnancy itself does not directly cause lymph node swelling, the immune system undergoes changes that can make it more sensitive to minor infections or inflammation. Swollen axillary nodes are typically benign and result from localized issues like irritation from shaving, a minor cut, or a skin infection such as folliculitis or an abscess. Swelling can also result from the body fighting a systemic infection, such as a cold or the flu, or an allergic reaction to a deodorant or soap.
A swollen lymph node typically feels like a firm, mobile mass, and it is often painful or tender, especially if it is actively fighting an infection. Pregnancy-related hormonal shifts can also increase fluid retention and blood flow, which may contribute to the enlargement of the lymph nodes. Though usually a sign of a routine immune response, any persistent or unexplained swelling warrants medical attention to rule out other causes.
Distinguishing Features and When to See a Doctor
Differentiating between a benign cause and a more serious one requires a professional assessment, but certain features can provide initial context. A lump caused by accessory breast tissue is typically soft, somewhat diffuse, and often feels connected to the main breast area. These lumps may also be bilateral and tend to be most prominent in late pregnancy or when milk production begins. Lumps from swollen lymph nodes are usually mobile, tender, and associated with signs of infection or recent trauma to the nearby skin.
Benign masses are typically soft, freely movable under the skin, and may fluctuate in size during the menstrual cycle or throughout pregnancy. These lumps often resolve after delivery or when the underlying infection clears. Other benign causes might include a lipoma (a soft, fatty tumor) or a sebaceous cyst, which are slow-growing and painless.
Features that necessitate immediate medical consultation include a lump that is hard, fixed, or immovable when pressed. These signs include a mass that grows rapidly, is persistently painless, or is accompanied by other symptoms like fever, unexplained weight loss, or skin changes such as dimpling or redness. Although pregnancy-associated breast cancer is rare, any new lump must be evaluated, as breast tissue changes during pregnancy can sometimes mask underlying conditions.
Evaluation usually begins with a physical exam and a history of the lump’s onset and associated symptoms. To confirm the nature of the lump, a doctor may order a breast ultrasound, which is safe during pregnancy and can effectively distinguish between solid tissue, a cyst, or an enlarged lymph node. If imaging results are suspicious, a biopsy may be performed to obtain a tissue diagnosis. All pregnant individuals should report any new or changing lump to their obstetrician or midwife to ensure a timely and accurate diagnosis.