Can Pregnancy Cause a Breast Cyst? Is Ultrasound Safe?

Yes, pregnancy and breastfeeding can cause fluid-filled breast lumps, including simple breast cysts and specialized milk-filled cysts. Because your breast tissue undergoes significant hormonal and structural changes during this time, physical exams become less reliable ^[1]. Therefore, any new or persistent lump should be evaluated promptly by a doctor rather than assumed to be a normal part of pregnancy or breastfeeding ^[2]. While most breast lumps during this time are not cancer, prompt assessment is important. Targeted breast ultrasound is usually the safest and most standard first test to evaluate these lumps, as it uses sound waves with no ionizing radiation ^[3].

Types of Breast Lumps During Pregnancy and Breastfeeding

Hormonal changes and milk production shift the types of lumps you might experience. If you discover a lump, your doctor will consider several possibilities:

• Galactoceles: These are milk-filled retention cysts that typically develop during or shortly after breastfeeding ^[4][5]. They are usually benign. While ultrasound often helps characterize them, a galactocele can sometimes look complex or solid; atypical features may require aspiration (draining the fluid with a needle), short-interval follow-up, or a biopsy to be certain ^[6].
• Simple Cysts: Standard fluid-filled sacs can still be detected during pregnancy ^[5]. While they are non-cancerous, a fluid-filled pocket must still be evaluated to confirm it is a simple cyst and not a more complex or concerning condition ^[6].
• Solid Benign Lumps (Fibroadenomas and Lactating Adenomas): Fibroadenomas are solid, benign (non-cancerous) tumors you may have already had, but they can grow rapidly and become tender during pregnancy due to hormones ^[7][8]. Lactating adenomas are another benign solid lump tied directly to pregnancy and lactation ^[9]. Any rapid change in a solid lump warrants checking ^[10].
• Mastitis and Abscesses: Mastitis involves inflammation of the breast tissue, which may or may not involve a bacterial infection. If it worsens, a pocket of pus called an abscess can form, which feels like a very painful, swollen lump and often requires drainage ^[11][1].

Safe Testing and Imaging

Navigating breast testing while pregnant or nursing can be stressful, but there are well-established, safe protocols. A typical evaluation includes arranging an appointment, undergoing imaging to see if the lump is fluid or solid, and then discussing follow-up or biopsy if the lump is not clearly benign.

• Targeted Ultrasound: This is usually the recommended first-line imaging test for a breast lump during pregnancy and lactation ^[3][12]. Ultrasound uses no ionizing radiation, meaning there is zero radiation exposure to your fetus or infant. It helps distinguish fluid from solid tissue, though it cannot always establish the exact cause without further testing ^[1]. Emptying the breast (pumping or feeding) before imaging may improve your comfort and the image quality.
• Diagnostic Mammography: Depending on your age and the ultrasound results, a mammogram may be recommended. Diagnostic mammography may be used when indicated during pregnancy; the fetal radiation dose is negligible with appropriate technique, and routine abdominal shielding is generally unnecessary ^[1][13].
• Image-Guided Biopsy: If a lump appears suspicious or cannot be definitively identified on imaging, a core needle biopsy (removing a tiny piece of tissue with a hollow needle) is the standard next step ^[14][15]. A biopsy is safe during pregnancy and lactation. Local anesthetics are generally compatible with breastfeeding, meaning you usually do not need to stop nursing. However, you should discuss the risks of bleeding, infection, and a small risk of a milk fistula (a complication where milk leaks from the biopsy site) with your procedural team ^[16].

When to Seek Prompt or Urgent Evaluation

Do not wait to see if a lump resolves on its own. While most lumps are benign, pregnancy-associated breast cancer can happen and sometimes mimics benign or inflammatory conditions ^[12][17].

Seek urgent, same-day medical evaluation if you experience any of the following signs of a severe infection or abscess:

• A fever (e.g., 38°C / 100.4°F or higher), chills, or feeling acutely unwell (systemic illness) ^[1][18]
• Spreading redness, warmth, or swelling of the breast ^[11][19]
• Rapidly worsening, severe pain ^[1]

Additionally, contact your clinician promptly for reassessment if:

• You notice non-infectious changes such as skin dimpling or thickening, a newly inverted nipple, spontaneous bloody discharge, or a new lump in your armpit ^[12][9]
• You are being treated for mastitis and your symptoms do not improve within 24 to 48 hours ^[1]