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Engorgement usually happens when the breasts switch from colostrum to mature milk (often referred to as when the milk "comes in"). However, engorgement can also happen later if lactating women miss several nursings and not enough milk is expressed from the breasts. It can be exacerbated by insufficient breastfeeding and/or blocked milk ducts. When engorged the breasts may swell, throb, and cause mild to extreme pain.
Engorgement may lead to mastitis (inflammation of the breast) and untreated engorgement puts pressure on the milk ducts, often causing a plugged duct. The woman will often feel a lump in one part of the breast, and the skin in that area may be red and/or warm. If it continues unchecked, the plugged duct can become a breast infection, at which point she may have fever or flu-like symptoms.
The first signs of the condition are the swollen, firm and painful breasts. In more severe cases, the affected breast becomes very swollen, hard, shiny, warm, and slightly lumpy when touched.
The condition may cause edematous and flushed nipples. In cases when the breast is greatly engorged, the nipple is likely to retract into the areola. Commonly, patients experience loss of appetite, fatigues, weakness and chills.
The symptoms of breast engorgement are similar to the symptoms caused by the Inflammatory Breast Cancer, for this reason it is very important ot seek medical attention if the condition does not completely clear within 2 weeks.
Breast engorgement can occur due to four main factors such as a suddenly increased milk production that is common during the first days after the baby is delivered or when the baby suddenly stops breastfeeding either because it is starting to eat solid foods or it is ill and has a poor appetite. Breast engorgement may also be caused when the mother does not nurse or pump the breast as much as usual.
After the first 3 to 4 postpartum days, the quantity of colostrum is quickly replaced by an increased milk production. When milk production increases rapidly, the volume of milk in the breast can exceed the capacity of the alveoli to store it and if the milk is not removed, the alveoli become over-distended which can lead to the rupture of the milk-secreting cells 
Accumulation of milk and the resulting engorgement are a major trigger of apoptosis, or programmed cell death, that causes involution of the milk-secreting gland, milk resorption, collapse of the alveolar structures, and the cessation of milk production.
Severe breast engorgement can lead to the flattening of the nipples or, it can result in inverted nipples which make it impossible for the baby to suck out all the milk from the breast. This is one of the common causes of the stagnation of milk in the breast.
Not all women experience breast engorgement after they give birth and some degree of engorgement of the breast is however normal within the few postpartum days. Women with mild to moderate hypoplastic breasts with a wide intramammary space (>1 inch) and a tubular shape are at particular risk for producing less than 50 percent of the milk necessary for the first week. More concerning are the moderate to severe degrees of breast engorgement. In these cases, the condition can continue for up to ten days or more even though the patients will experience serious symptoms only during the first six days.
Overfilled breasts can lead to severe engorgement due to waiting too long to begin breastfeeding the baby, not feeding often enough or due to small feedings that do not empty the breast, very common in cases when the baby is fed formula or water. Severe engorgement of the breast can lead to breast infection.
Regular breastfeeding should be continued, supportive treatment and hot and cold compresses can help.
Wearing a well fitting maternity bra with wide straps that do not scratch and with a cup that comfortably holds the entire breast usually help in easing the discomfort and other symptoms.
If the symptoms do not improve after a few days, the patient is advised to seek a doctor.
-  What is breast engorgement, and what causes it?
-  LLLI Breastfeeding and Engorgement
- Mangesi, L.; Dowswell, T. (2010). Mangesi, Lindeka, ed. "Cochrane Database of Systematic Reviews". Cochrane database of systematic reviews (Online) (9): CD006946. doi:10.1002/14651858.CD006946.pub2. PMID 20824853.
-  Breast Engorgement - Retrieved 29-03-2010