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Some Women Have a Harder Time Maintaining A Milk Supply -- That’s Just Biology

My obstetrician used to tell me that if I had been born two hundred years ago, with my ability to maintain a milk supply, I would have been a wet nurse. 

But I am not the rule. If anything, I am probably the exception. Many women have trouble maintaining milk supply at some point. And this fact is not about effort, parenting skills, or love for your baby. It is just biology.

Some women are distance runners. Others are sprinters. Some woman can belt out a tune. Others (like me) can’t sing Row, Row, Row Your Boat without going off key. And some women have to work harder than others–pump or nurse more often–to make the same amount of milk.


Some women have to work harder than others–pump or nurse more often–to make the same amount of milk.


This is true even when your nursing relationship is going well. It’s true even after you have passed through all the early breastfeeding hurdles with flying colors–or perhaps with a few bumps in the road, but you still finally made it. Your milk came in. Your baby is latching well and gaining weight. Your nipples are no longer feel like they’re being flayed by Ramsey Bolton. Even after skirting these common pitfalls, some women still have a harder time maintaining a milk supply once breastfeeding has been established. And some women have a easier time.

Milk Supply Myths and Judgement Traps

For women like me, who were–let’s be clear–lucky, it can prove all too short of a hop to judgement and knee jerk criticism of other moms who are less lucky – “They are just not trying hard enough”

It’s an easy trap to fall into, because so many new moms-to-be hear falsehoods that promote judgement of mothers who struggle with breastfeeding, such as:



    All mothers can breastfeed.



    Some mothers never start producing milk after giving birth, for a variety of reasons, including insufficient glandular tissue, Polycystic Ovary Syndrome (PCOS), breast surgery, or Sheehan’s syndrome)





    All mothers can produce enough milk for their babies.



    A recent study of mothers seeking help for breastfeeding problems found that a 2 out 3 had inadequate milk supply 11-13 days after birth (defined as less than 440 mL per day), and 1 out 3 continued to have inadequate supply by the end of the first month.)



    All women can easily maintain an adequate milk supply.



    Some women have to work much harder to produce enough milk.

The truth about milk supply

Let’s focus in on that last point. This fact boils down to biology: Women vary dramatically in their breast storage capacity–how much milk their breasts can hold.

The range in breast storage capacity from woman to woman is enormous. In one small study of 13 women, the per breast storage capacity ranged from a meager 2.6 ounces to a whopping 20 ounces–an almost a tenfold difference from breast to breast. Even for the same woman, storage capacity often differed from left breast to right breast.

18 oz of pumped breast milk
Milk from a single pumping session of a mother who was exclusively breastfeeding her 3-month-old twins. Double the emptying = lots of milk.

(Fun fact: For unknown reasons, about two-thirds of time a woman’s right breast has a higher storage capacity than her left.)


Even for the same woman, storage capacity often differs from her left breast to her right.


In another study of exclusively pumping mother, women with low storage capacities typically pumped 5 ounces or less first thing in the morning, whereas women with high storage capacities routinely pumped over 10 ounces.

A clue about your milk storage capacity

Lactation consultant, Nancy Mohrbacher, offers a great tool for figuring our your individual milk storage capacity. Dubbed a your “magic number”, it accounts for the number of times in a 24-hour period you typically nurse or pump. (There will be some variance, because babies sometimes nurse more often during a growth spurt.) According to Mohrbacher, for mothers of exclusively breastfed babies between 1 to 6 months of age, 8 is the average magic number. If your number is higher than this, you likely have a lower than average storage capacity. Conversely, If your number is lower than this, you likely have a higher than average storage capacity–lucky you!

“Due to differences in breast storage capacity, some mothers’ “magic number” may be as few as 4-5 or as many as 9-10. But when a mother’s total number of breast drainings (breastfeedings plus milk expressions) dips below her “magic number,” her milk production slows.”

– Nancy Mohrbacher

(Side note to an ex who said he preferred busty women because he wanted his children to be “well fed”: A woman’s breast size is mainly determined by the amount of fatty tissue. It has nothing to do with her breasts’ storage capacity. Storage capacity is determined by the amount of glandular tissue.)

Why milk storage capacity matters

Body-wide hormonal changes during pregnancy and delivery drive the initial production of breastmilk (your milk “coming in”). But after your milk comes in, your milk supply shifts to being locally controlled. Now, the filling and emptying of your breasts regulates your milk supply.

Simply put: full breasts mean less milk; empty breasts mean more milk.

When your breasts are empty, prolactin enters into the alveoli and binds to its receptors there, stimulating milk production. When your breasts are full, the milk blocks the binding of prolactin. At the same time, a chemical in the milk, known as Feedback Inhibitor of Lactation, signals to your breasts to slow down milk production.

Because women with high storage capacities have breasts that fill up relatively slowly, they do not need to pump or nurse as often to keep their breasts from filling up and dropping their supply.


full breasts mean less milk; empty breasts mean more milk.


The upshot: Some women notice a drop in their milk supply after weaning from night feedings. Others pass through this magical milestone without a hitch. Some women need to feed their 3-month-old baby round the clock. Others can go for a 5 to 7 hour stretch without nursing. And some lucky mamas get by only pumping once or twice a day while at work, while others need to pump three or four times a day.

To be clear, your individual milk storage capacity is not about your ability to feed your baby. Women with low storage capacities can produce enough milk for their babies. They just have to work a lot harder at it.

So let’s stop with the judgement, spread the word, and give some extra love and support to the mamas working their butts off to maintain their milk supply.


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About Amy

Amy Kiefer is a researcher by training, and earned her Ph.D. from the University of Michigan. She currently lives in the Bay Area with her husband and three children where she writes about fertility, pregnancy, and breastfeeding. Check out her blog,, for more great evidence-based pregnancy and parenting info.


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