A blocked duct is an area of the breast where milk flow is obstructed. The nipple pore may be blocked (see Milk Blister), or the obstruction may be further back in the ductal system. A blocked duct usually comes on gradually and affects only one breast.
Mom will usually notice a hard lump or wedge-shaped area of engorgement in the vicinity of the plug that may feel tender, hot, swollen or look reddened. Occasionally mom will only notice localized tenderness or pain, without an obvious lump or area of engorgement. The location of the plug may shift. A blocked duct will typically feel more painful before a feeding and less tender afterward, and the blocked area will usually feel less lumpy or smaller after nursing. Nursing on the affected side may be painful, particularly at letdown.