Milk ducts are a key part of lactation. In the breast, the ducts connect the milk-producing glands, also known as mammary glands, with the nipple. The milk is contained within lobes in the mammary glands. It then passes through the ducts to the lactiferous sinus, a dilation that stores the milk at the tip of the nipple until it is expelled. Other names for milk ducts include lactiferous ducts, galactophorous ducts, and mammary ducts.
The mammary lobes are situated in the central mass of the breast, connected by a branching network of lactiferous ducts. During lactation — usually induced by childbirth — the glands begin to produce milk. As this passes into the ducts, they converge towards the nipple. The milk gathers in the enlarged area of the lactiferous sinus, which rests between the main channels of the milk ducts and the smaller channels through which milk leaves the nipple. When suction or pressure is applied to the nipple during nursing, milk squeezes from the lactiferous sinus through the last of the duct and exits the nipple.
When an obstruction forms in any area of the primary channel or at the nipple pore, milk ducts can become clogged. If the nipple pore is not properly cleaned, milk can build up within the opening, dry up, and create a plug that blocks the opening. Sometimes a thin layer of skin will grow over the nipple pore, blocking it and creating a blister that fills with milk. Clogs can also build up over time deeper within the duct. Obstructions can be caused by feeding improperly or not often enough, ill-fitting nursing bras, incorrect use of a breast pump, or illness.
More severe cases of milk duct obstruction can lead to a condition known as mastitis, which is an infection of the breast. Mastitis may be caused by aggravated or extended obstructions, which can become infected. The condition can also arise from bacteria entering the body through the nipple pore, either during feeding or some other kind of contact. Without antibiotics the infection can persist and can even taint breast milk.
Inflammation, soreness, hard areas of breast tissue, and tender lumps in the breast can all be signs of clogged milk ducts. In the case of mastitis, these symptoms may be accompanied by fever, chills, or fatigue. Usually these signs occur only in one breast, unless the lactating female suffers obstructions caused by improper draining or feeding in both breasts.