The fuller story
Size and supply come apart because of what a breast is made of. Toronto Public Health’s guidance for health professionals puts it plainly: storage capacity “does appear to be related to breast size but not to the potential for breast milk production.” Larger breasts hold more fat, not necessarily more of the milk-making glands. An ultrasound study of 21 lactating women found no correlation between milk production and the amount of glandular tissue. What storage size can change is rhythm rather than total: someone who stores less may simply nurse or pump more often.
Left and right rarely match, either. A 2006 study in Pediatrics found that left and right breasts rarely produced the same volume of milk. In another study, mothers pumping both sides at once for very premature babies got more from the right breast in about two out of three observed sessions.
Shape follows the same rule. A 2020 study of 54,591 mammograms from a UK screening program measured the typical difference in volume between the two breasts at around two to three percent, with the left larger on average. Only 51 of those women had a difference of more than 20 percent. (The women were mostly in midlife and not nursing, so think of these as resting numbers.)
Why it matters
The book has a line for this: when it comes to the human breast, it’s all about performance, not packaging. The research agrees. A small breast isn’t a small promise, and a mismatched pair is just a pair.
Every body is the right body. That’s the spirit of The Memory of Milk, too: the biology beneath tenderness is generous, and it doesn’t check cup sizes. For questions about supply, an IBCLC is the person to ask.
