A plugged milk duct is caused by a persistent narrowing of the complex, interlocking ductal system within the breast that serves as the conduit for milk from the mammary glands to the nipple.

The etiology of this narrowing is most often from sequelae of milk stasis from mechanisms such as milk overproduction, improper drainage, or external compression. Milk stasis leads to edema and increased blood flow to the area, which subsequently narrows the ducts and causes a "plug." When milk cannot drain properly, it results in alveolar distention, inflammation, and pain.