What causes breast engorgement?

Primary breast engorgement is caused by hormonal shifts that cue the production of copious amounts of milk, and these shifts are called lactogenesis II or the second stage of milk production. During this stage, there is a rapid drop in progesterone and a rise in prolactin, cortisol, and insulin, which stimulate breast milk to “come in” and replace colostrum (the “first milk”).^[1] This sudden increase in milk production and interstitial fluid volume causes breast swelling and the signs and symptoms of engorgement. When no complications are present and feeding techniques are effective, it is a self-limiting condition that resolves within the first week postpartum as milk production regulates to match infant demand.^[2]

Secondary breast engorgement occurs later in the postpartum period when there is a mismatch between milk production and milk removal.^[3] There are a wide variety of reasons why this might happen, some related to the lactating parent, some to the infant, and some to both. Examples include a hormonal imbalance in the parent that increases milk production (like hyperthyroidism), pumping too frequently and stimulating excessive milk production, breastfeeding difficulties (like poor latch or positioning), and decreased infant feeding (maybe due to weaning, an infant sickness, or the infant sleeping for longer periods of time at night). Identifying the underlying issue is integral to finding the appropriate treatment.^[4]