Your Period Contains More Information Than We Thought

So, what exactly is in menstrual blood?

Calling it “blood” is technically correct but... it is also slightly misleading.

Menstrual fluid is not simply the same blood you would find in a tube taken from your arm.

As the endometrium (the tissue lining the inside of the uterus) breaks down during menstruation, its contents leave the body together with blood from small vessels, cervical and vaginal secretions and fragments of endometrial tissue.

This means a menstrual sample can contain red blood cells, epithelial and stromal cells from the endometrium, immune cells, proteins, inflammatory molecules, metabolites, DNA, RNA and tiny membrane-bound particles called extracellular vesicles.

In other words, menstrual fluid contains cells and fragments from the endometrium itself, which is exactly the tissue researchers are often trying to study. And that is what makes it so interesting.

Traditionally, if researchers wanted to study the endometrium up close, they often had to collect tissue directly from inside the uterus with a biopsy. Menstruation offers a much less invasive window into the same tissue, because the endometrium is already shedding cells and fragments every month.

The uterus leaves clues behind

The endometrium is a pretty unusual tissue.

Every cycle, it grows, changes and prepares for the possibility of an embryo. If pregnancy doesn’t happen, part of that tissue breaks down and is shed during menstruation, before the whole process starts again. Repeating that cycle month after month takes an impressive amount of coordination between hormones, blood vessels, immune cells and the endometrial cells themselves.

Hormones change gene activity. Blood vessels remodel. Immune cells move through the tissue. Inflammatory signals help coordinate breakdown and repair. Stromal cells change their behaviour in preparation for implantation.

When menstruation begins, some of that molecular information leaves the uterus with the menstrual fluid.

Researchers can use menstrual fluid to look at broader patterns; things like gene activity, proteins, immune cells, lipids and extracellular vesicles to get a better sense of what was happening inside the endometrium before it was shed.

That makes a period unexpectedly useful: a naturally recurring, relatively non-invasive look into the uterine environment.

And yes, there are immune cells in your period

I know... this might not be the first thing that comes to mind when you think about menstruation, but your period is also an immune event!

As the uterine lining breaks down, the body has to clear away tissue, control bleeding and start repairing the endometrium almost straight away. And guess what... Immune cells are involved throughout that process, helping manage inflammation and organise the clean-up!

Some of those immune cells and signalling molecules end up in menstrual fluid.

In conditions such as endometriosis, the immune environment of the uterus appears to behave differently, and scientists are investigating whether those differences could eventually be picked up in menstrual samples.

So something we usually think of simply in terms of bleeding, cramps and getting through the week may also be carrying a record of what has been happening inside the uterus.

Endometriosis is where this starts to get really interesting

Endometriosis is one of the clearest reasons scientists are paying such close attention to menstrual fluid.

Part of the problem is that endometriosis can be frustratingly difficult to diagnose.

Symptoms vary widely, scans can miss certain forms of the disease, and there is still no menstrual-blood test used routinely in clinics that can simply confirm, with confidence, that someone has it.

That makes menstrual fluid an interesting place to look, because it comes directly from the reproductive environment that is affected by the condition.

Researchers have started comparing menstrual samples from people with and without endometriosis, looking for biological patterns that might distinguish the two.

A 2026 systematic review pulled together 32 studies and found a number of promising candidates, including markers linked to aromatase activity, TGF-β1, IGFBP1 and progesterone signalling.

Some of those studies reported impressive diagnostic performance. But this is also where the science needs a little restraint, because most of the evidence is still early.

Many studies are small, menstrual samples are not always collected in the same way, laboratories use different methods, and the cut-offs for what counts as an “abnormal” result are not yet standardised.

A biomarker can look very convincing in one research group and still fail when it is tested in a larger, more diverse population.

So we are not at the point where a period sample can reliably diagnose endometriosis. But we may finally be looking in the right place. And honestly, this gives me another reason to be proud to be a scientist.

What if your cells have been leaving us messages?

Extracellular vesicles are one of those things that make cell biology feel almost surreal. They are tiny membrane-bound particles released by cells, carrying proteins, lipids and different forms of RNA from one place to another.

In simple terms, they are one of the ways cells communicate.

And yes, menstrual fluid contains them too.

What makes them interesting is that they can carry molecules from endometrial cells. By studying what is inside them, researchers may be able to learn more about what is happening in the uterus.

In 2025, scientists studying small extracellular vesicles in menstrual fluid identified thousands of proteins and found differences between samples from women with and without endometriosis.

Some of those differences involved pathways linked to immune signalling, cell death and the way cells maintain their structure.

Another study that same year looked at extracellular vesicles in menstrual blood from women with unexplained infertility, asking whether their molecular profiles might reveal biological differences in cases where standard investigations had not identified a clear cause.

We are still very early in this research, so these findings are nowhere near ready to become routine diagnostic tests.

But the idea itself is beautiful. Being a woman myself, I think that is what makes this research hit differently.

Endometriosis can still take years to diagnose, and for some women getting a clear answer can involve laparoscopy and surgery.

So the thought that menstrual fluid (something we usually just throw away without a second thought) might one day help doctors understand what is happening inside the uterus feels incredibly important.

We are not there yet, and I do not want to pretend that we are. But the possibility of getting women answers sooner, with less waiting and fewer invasive procedures, is exactly the kind of science I want to see more of.

Your Period Contains Cells That Can Keep Growing in the Lab

Menstrual fluid also contains stromal cells with stem-like properties, often called menstrual blood-derived stem/stromal cells, or MenSCs. In simple terms, these are cells that can keep dividing and can develop some of the behaviours we associate with tissue repair and regeneration.

They are especially interesting because the endometrium is constantly rebuilding itself.

Scientists can isolate these cells from menstrual samples and grow them in the lab to study how they behave.

In endometriosis research, menstrual-derived stromal cells from affected women have shown differences in how they grow and invade surrounding environments.

Other research also points to changes in pathways involved in cell survival.

Could your period become part of cervical screening?

Menstrual fluid passes through the cervix and vagina on its way out of the body, it can also collect material from those tissues (including DNA from human papillomavirus, or HPV). That has led researchers to ask whether menstrual blood could one day become another way for women to collect their own samples for HPV testing.

A 2026 review looked at seven studies involving 1,672 women. Menstrual-blood testing picked up a high proportion of women with high-risk HPV-associated cervical abnormalities, but it also produced a relatively high number of positive results in women who did not have those abnormalities. The studies also differed considerably from one another, which makes the results harder to generalise.

Well... your period is not about to replace cervical screening... but I do find the idea interesting.

If researchers can eventually make menstrual sampling accurate and reliable enough, it could offer another self-sampling option for women who find conventional screening difficult, uncomfortable or simply avoid it altogether.

What if the pad became part of the test?

This is probably the part that sounds the most futuristic.

In 2025, researchers developed a proof-of-concept system called MenstruAI, which built a small diagnostic test directly into a sanitary pad.

As menstrual blood passed through the pad, the device was designed to detect certain biomarkers through a colour change.

The researchers tested markers including C-reactive protein, CEA and CA-125, and the result could be read by eye or analysed using a smartphone.

To be very clear, this does not mean a sanitary pad can currently diagnose cancer, endometriosis or inflammation.

It was a research prototype.

But the idea behind it is what I find exciting. Instead of asking someone to book an appointment, have blood taken and capture one single moment in time, menstrual products could potentially collect biological information during something that is already happening naturally! How exciting!

And because periods happen repeatedly, researchers are also interested in the possibility of following changes over time rather than relying on one isolated measurement.

Finding a molecule is the easy part. Knowing what it means though is harder.

This is where I think we need to be careful not to get carried away.

Menstrual blood is biologically rich, but that also makes it complicated.

What ends up in a sample can depend on when during the period it was collected, how it was collected, the person’s hormones, medications, contraception and probably many other factors researchers are still trying to understand.

Even the collection method matters. A sample taken from a menstrual cup is not necessarily identical to material recovered from a pad.

And finding a molecule that looks different in people with a disease is only the beginning.

Scientists then have to show that the difference appears consistently, in much larger groups of people, across different laboratories and in patients who actually resemble the people doctors see every day.

And that is exactly the gap between an interesting biomarker and a useful medical test. And t is a very big gap I am afraid...

Maybe we were never looking closely enough

Blood gets analysed. Urine gets analysed. Saliva gets analysed. We even analyse stool.

And yet every month, millions of women produce another biological sample containing blood, cells from the endometrium, immune signals, proteins and genetic material... and for most of medical history, we have barely asked what we could learn from it.

That is beginning to change.

Researchers are now looking at menstrual blood as a possible source of information about reproductive health, inflammation and disease, and even as a sample that might eventually allow certain measurements to be followed over time.

I do not think the exciting part is that your period is secretly a diagnostic test.

The exciting part is that something so ordinary, something women have been dealing with every month for generations, may contain far more useful biology than medicine ever bothered to look for.