It is extremely common for breastfeeding parents to notice a difference in milk production between their two breasts, a phenomenon sometimes called “slacker boob.” This asymmetry is a normal variation of human anatomy and function, and it rarely indicates a failure in overall milk supply. Many parents successfully breastfeed for months or years even with one breast producing significantly more milk than the other.
The Reality of Uneven Milk Supply
Uneven milk supply is the term used when one breast consistently produces less milk than the other, which is often noticed during pumping sessions. Breasts are independent organs, meaning that perfect symmetry in their capacity to produce milk is actually unusual. While breast size differences are often noticeable, size is determined mostly by fatty tissue, which does not produce milk. The milk-making ability is tied to glandular tissue, and this tissue can vary in amount between the two sides.
It is common and manageable for one breast to produce 20% to 30% less milk than the other without affecting the baby’s growth. When overall milk production is sufficient, the higher-producing breast often compensates for the lower output of the other side. This difference only becomes a concern if the total output is not enough to meet the infant’s nutritional needs.
Biological and Behavioral Causes for Differences
The reasons for a production difference fall into two main categories: anatomical differences and the principle of supply and demand. Anatomical differences originate from the physical structure of the breast. One breast may contain naturally less glandular tissue than the other, which can be a result of uneven development during puberty. Previous breast surgery, such as augmentation or reduction, can also affect supply by removing glandular tissue or damaging the nerve pathways necessary for the milk ejection reflex, or letdown.
Sometimes, a history of recurring issues like a plugged duct or mastitis on one side can cause localized damage or inflammation that reduces the efficiency of the milk-making tissue. Differences in the size or shape of the milk ducts between the two breasts can also affect how efficiently milk is removed, which then signals the body to produce less.
The most frequent cause, however, is the behavioral principle of supply and demand, where milk production increases with stimulation and removal. Infants often develop a preference for one breast due to reasons like an easier latch, a faster or slower milk flow, or physical comfort from a preferred feeding position. When a baby consistently nurses more effectively or for longer periods on one side, that side receives more stimulation and, consequently, produces more milk. Similarly, consistently starting pumping sessions on one breast or using one side more frequently inadvertently creates an imbalance by signaling greater demand on the favored side.
Techniques for Managing Supply Asymmetry
Parents who wish to balance their milk supply can implement strategies focused on increasing demand on the less productive side. A simple yet effective approach is the “Power Side” technique, which involves always offering the lower-producing breast first at the start of a feeding. Babies typically suckle most vigorously at the beginning of a feed, and this intense stimulation can help encourage greater milk production on that side. Changing feeding positions can also help, as certain holds may make it easier for the baby to latch and drain the less-preferred breast more effectively.
When pumping, parents can add an extra five to ten minutes of stimulation on the lower-producing side after a full session to signal increased demand. Gentle breast massage and compression during both nursing and pumping can also improve milk flow and removal from the “slacker” side.
Achieving perfect symmetry is not always possible or necessary for a successful breastfeeding journey. If the difference in size is sudden, accompanied by pain, redness, a hard lump, or if the infant completely refuses one side, a consultation with an International Board Certified Lactation Consultant (IBCLC) or a healthcare provider is recommended. These professionals can help identify underlying issues and create a personalized plan to manage the asymmetry.