Mammary duct ectasia happens when a milk duct beneath the nipple widens and fills with thick, sticky fluid. It is most common in women aged 40–60 and can mimic more serious breast problems, so knowing its typical signs is helpful.

The classic first complaint is a sticky, greenish, brown or black nipple discharge that may stain a bra or be noticed only when the nipple is squeezed. The discharge is usually painless and varies from a few drops to a constant moistness. Many women feel a dull, heavy ache or a burning sensation behind the nipple that can radiate into the upper outer breast or even the armpit. The pain tends to be mild and cyclic, but it can flare for several days and then ease.

A firm, tender lump is often found near the areolar edge. This lump is caused by the swollen duct and surrounding inflammation; it may come and go and is sometimes mistaken for a cyst. Overlying skin can look pink or feel warm, and the nipple may be pulled inward (a new, painless inversion) because the stretched duct shortens as it scars. If the stagnant fluid becomes infected, the area becomes red, swollen and more painful, and a low-grade fever can develop; this phase is often called periductal mastitis.

Symptoms usually settle within weeks, but thick discharge or nipple retraction can persist. Any spontaneous bloody discharge, fixed hard mass or progressive nipple changes should prompt further evaluation to rule out other conditions.