Mastitis is common, especially among breastfeeding women. Estimates suggest that up to 20 to 25% of breastfeeding women will develop at least one episode during the course of lactation, making it one of the most frequent complications of breastfeeding. The condition also affects people who aren’t breastfeeding, though reliable numbers for that group are harder to pin down.
Rates in Breastfeeding Women
Studies from multiple countries consistently place lactational mastitis somewhere between 10% and 25% of breastfeeding women, depending on the population studied and how mastitis is defined. A large cohort study in China found a rate of about 10%, while observational research from the United States, Finland, New Zealand, and Australia puts the figure closer to 20 to 25%. The variation partly reflects differences in breastfeeding support, how long women nurse, and whether mild cases get counted alongside severe ones.
These numbers likely undercount the real burden. A study published in Pediatric Research found that subclinical mastitis, a form with no obvious symptoms but measurable inflammation in breast milk, was present in 23% of breastfeeding women. These women wouldn’t know they had it without lab testing, which means the total number of women experiencing some degree of breast tissue inflammation during lactation is higher than clinical estimates suggest.
When It Typically Happens
Most episodes cluster in the first three months after delivery, which is when milk supply is still regulating and latch issues are most common. But mastitis is not exclusively an early postpartum problem. Research on long-term breastfeeding found that about one-third of episodes occurred after six months, and nearly a quarter happened after a full year of breastfeeding. If you’re nursing past infancy, the risk doesn’t disappear.
Recurrence Is Surprisingly Frequent
Once you’ve had mastitis, there’s a meaningful chance it will come back. In a prospective study from Glasgow, 23% of women who developed mastitis reported two separate episodes, and 9% had three or more during a single period of breastfeeding. Broader data across multiple studies suggests that 20 to 35% of women who get mastitis will experience a recurrence. Recurrent episodes often point to an underlying issue like persistent latch problems, incomplete milk removal, or disruption of the normal bacterial balance in the breast.
The Mastitis Spectrum
Mastitis isn’t a single condition. The Academy of Breastfeeding Medicine now describes it as a spectrum of related problems, each representing a different stage or type of breast inflammation.
At the mildest end, ductal narrowing causes localized pain and reduced milk flow without infection. If inflammation progresses, it becomes inflammatory mastitis, which brings redness, swelling, and systemic symptoms like fever and chills. Importantly, this stage can look and feel like an infection even when no bacteria are involved. The body’s own inflammatory response is enough to cause fever and flu-like misery.
Bacterial mastitis develops when bacteria, most commonly Staphylococcus and Streptococcus species, take hold in the inflamed tissue. This stage typically requires treatment to resolve. There’s also a chronic form called subacute mastitis, where bacteria form films inside the milk ducts. Subacute mastitis causes milder, ongoing symptoms like deep breast pain and reduced milk output without the dramatic redness and fever of acute episodes. It’s often missed or misdiagnosed because it doesn’t look like “classic” mastitis.
Progression to Abscess
The main concern with untreated or poorly managed mastitis is the formation of a breast abscess, a walled-off pocket of pus that typically requires drainage. About 2 to 3% of breastfeeding women develop mastitis, and of those, 5 to 11% go on to develop an abscess. In practical terms, that means the overall risk of abscess during breastfeeding is low, but it’s a real possibility for women whose mastitis doesn’t respond to initial treatment. Persistent fever, a firm lump that doesn’t improve, or worsening pain after several days of treatment are signs that an abscess may be forming.
Mastitis Outside of Breastfeeding
Non-lactational mastitis is less well studied, and reliable prevalence figures are scarce. Most published research focuses on breastfeeding-related cases, and the studies that do exist on non-lactational forms tend to be small with significant methodological limitations. What is clear is that it occurs most often in women of reproductive age, whether or not they are currently breastfeeding. Men can also develop mastitis, though it’s rare.
The bacterial causes differ somewhat from lactational mastitis. Staphylococcus remains the most common culprit, but up to 30% of non-lactational cases involve multiple types of bacteria. In certain regions, tuberculosis is a notable cause. A systematic review of non-lactational infectious mastitis cases in the Americas found that tuberculosis accounted for 38% of confirmed bacterial diagnoses, followed by Corynebacterium species at about 21%. Fungal infections made up roughly 10% of cases. These patterns vary significantly by geography and access to healthcare.
Why These Numbers Matter
Mastitis is common enough that experiencing it says nothing about your ability to breastfeed or your hygiene. It happens to roughly one in four to five breastfeeding women regardless of preparation or intent. Understanding this can help set realistic expectations: if you develop a painful, red area on your breast with or without fever in the weeks or months after delivery, you’re dealing with something that affects hundreds of thousands of women every year. Early recognition and management, particularly maintaining milk removal from the affected breast, remain the most effective ways to prevent a mild episode from progressing to something more serious.