Low Milk Supply Causes: The Biology Behind Why 'Just Pump More' Doesn't Always Work

By Ece · Founder of Idaho Jones, pumping mom
Published May 2025 · Reviewed May 2026

If you have ever sat in those early weeks, exhausted and worried that your baby is not getting enough, only to be told to simply nurse more or pump more often — this post is for you.

Because science is catching up to what so many women have quietly suspected for years: sometimes, the issue runs deeper than technique. And it has nothing to do with how hard you are trying.

The numbers have been wrong

For decades, medical guidance has operated on the assumption that true low milk supply — meaning a biological inability to produce sufficient milk — affects only around 5% of women. On that basis, the standard advice has been behavioural: feed more, pump more, stimulate more.

But recent research from the US and Australia is painting a very different picture.

That is not a small number. That is millions of women who have been told to try harder, when the answer lay in their biology.

The cell at the centre of it all: meet the lactocyte

To understand why supply problems occur, you need to know about lactocytes — the specialised cells inside the mammary glands that actually produce breast milk. During pregnancy, lactocytes multiply rapidly to prepare the breast for feeding. Around three days after birth, they begin secreting milk in earnest.

But not all lactocytes are created equal, and not all women end up with the same number or quality of these cells. Research has identified two main ways things can go wrong:

• Too few lactocytes. Some women simply do not develop enough milk-producing cells during pregnancy — a structural issue that no amount of extra nursing or pumping can compensate for.
• Inefficient lactocytes. Some women have the cells, but those cells do not secrete milk at full capacity — often due to inflammation, nutrient deficiencies, or disrupted hormonal signalling.

Both of these are biological realities, not behavioural failures.

The WEE1 discovery: why some women have fewer lactocytes

One of the most significant recent breakthroughs came from Dr Lindsay Hinck's lab at UC Santa Cruz. Her team's research, published in Nature Communications in April 2024, identified a key cellular mechanism governing how many high-functioning lactocytes a woman develops during pregnancy.

During pregnancy, mammary cells divide rapidly — and some undergo a special process called endoreplication, where a cell copies its DNA multiple times without splitting into two separate cells. The result is a polyploid cell packed with extra genetic material, making it significantly more productive at secreting milk.

This process is regulated by an enzyme called WEE1. When WEE1 functions correctly, it halts cell division at just the right moment, allowing these high-output cells to form. When WEE1 is deficient or malfunctions, fewer productive cells develop — and milk supply suffers.

The experiments were conducted in mice, but the implications for human lactation are significant. Understanding this pathway opens the door to potential treatments that could increase the proportion of high-functioning lactocytes before or during lactation.

When inflammation gets in the way

Even when a woman has healthy lactocytes, those cells need the right raw materials to do their job. Fatty acids are essential — both as an energy source for the cells themselves and as key building blocks in breast milk fat.

Research led by Dr Rachel Walker at Penn State University found that chronic inflammation may block the mammary gland's ability to absorb fatty acids from the bloodstream. Her team compared women with very low, moderate, and normal milk production — and found that the very low supply group had significantly higher inflammation markers and obesity rates, as well as lower levels of long-chain fatty acids in their milk. The study was published in the Journal of Nutrition in 2022.

The mechanism involves an enzyme called lipoprotein lipase (LPL), which transports fatty acids from the blood into the mammary gland. Inflammatory molecules — particularly TNF-alpha, elevated in obesity and autoimmune conditions — suppress LPL activity, cutting off the nutrients that lactocytes need to function.

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Other biological risk factors worth knowing

Placental health

Many of the hormones that trigger lactocyte development during pregnancy are produced by the placenta. Researchers are currently studying whether placental damage or infection is connected to supply problems — and whether low supply risk might one day be identifiable at the moment of birth. Pre-eclampsia, which affects around 4% of pregnancies, is already associated with supply difficulties in a significant proportion of cases.

Zinc transporter gene variants

Some women carry mutations in a gene called SLC30A2, which affects how zinc moves from the blood into breast milk. Zinc is not only critical for infant development — it also plays a role in mammary gland function and milk secretion. Research going back to 2006 has linked these mutations to low milk zinc concentrations and related supply difficulties.

Breast anatomy

Women with widely-spaced breasts tend to have less glandular tissue overall, meaning fewer lactocytes to begin with. This is not a modifiable factor — and yet it is rarely discussed in breastfeeding consultations.

Better diagnostic tools are coming

One of the most significant gaps in current breastfeeding care is the absence of reliable diagnostic tools for low supply. Without a test, clinicians have defaulted to behavioural advice — because that is all they have had. That is beginning to change.

Researchers have been developing handheld devices capable of measuring sodium levels in breast milk — a marker of how well the mammary gland's secretory process is working. When lactation functions normally, the junctions between cells lining the mammary gland close tightly, preventing sodium from leaking into the milk. Elevated sodium signals disruption. A smartphone-compatible sensing device tested with over 590 real-world mothers showed strong correlation between conductivity readings and sodium levels.

A 2025 study from the University of Western Australia also used machine learning to analyse milk composition data from 460 lactating women, identifying markers predictive of low milk supply. These tools are not yet standard practice, but they represent meaningful progress toward a future where low supply can be diagnosed — not just suspected.

What this means for you

If you have struggled with low milk supply, or are navigating it right now, here is what this growing body of research tells us:

• Low supply is far more common than previously understood, and a significant proportion of cases have a biological cause.
• Conditions like obesity, diabetes, pre-eclampsia, and autoimmune disorders are genuine risk factors — not incidental ones.
• Your anatomy, your placenta, your inflammatory profile, and your genetics all play a role that nursing frequency cannot override.
• Better diagnostic tools are being developed, and with them, more tailored advice and eventually targeted treatments.

You were not failing. Your body was navigating something genuinely complex, without the tools or understanding it deserved.