Why the Vaginal Microbiome Changes Everything We Thought We Knew About Period Care

For nearly 90 years, tampons have been designed without a single thought for the ecosystem they enter.

At The Daughters of Mars we started asking a simple question: what would a tampon look like if it were designed with this ecosystem in mind?

Here’s what that ecosystem actually does - and why ignoring it has never been a neutral act.

The ecosystem nobody talks about

You have probably heard about the gut microbiome. You may have changed your diet because of it, taken probiotics, read the articles...But there is another microbial ecosystem in the female body - one that is just as critical to health, far less understood, and almost entirely absent from mainstream conversation.

The vaginal microbiome. And the silence around it is not accidental.

It is part of a much older pattern: the systematic underfunding, under-research and under-communicating of women’s biology. Only a very small fraction (around 2%) of global medical research funding goes towards reproductive and gynaecological health (despite the fact that it affects over half the global population).^¹

The result is that most women reach adulthood without ever learning that their vagina is home to a finely tuned microbial system that protects them every single month and that the products they’ve been handed to manage their period may be working against it.

A different kind of ecosystem

The gut microbiome is vast and complex...a galaxy of thousands of bacterial species in constant negotiation with one another. The vaginal microbiome works differently.

In its healthiest state, it is defined not by diversity, but by dominance. A small number of Lactobacillus species hold the ecosystem in balance, producing lactic acid that keeps vaginal pH low (ideally between 3.5 and 4.5) creating an environment that is inhospitable to harmful bacteria and pathogens.^²

Some species produce hydrogen peroxide and antimicrobial compounds, adding further layers of defence. This elegant simplicity is what makes the vaginal microbiome so effective - and so vulnerable. When the balance shifts, the whole system becomes susceptible.

Scientists have known about these protective bacteria since 1892, when German gynaecologist Albert Döderlein first described them. But it is only in the last two decades, with advances in genetic sequencing, that we have begun to map the full picture of the vaginal microbiome and to understand how profoundly its disruption shapes women’s health across their lifetimes.

 

Why balance matters more than you might think

Infection

When Lactobacillus populations are displaced, the most common result is bacterial vaginosis (BV), an imbalance where protective bacteria are crowded out by other microbes. BV affects one in three women of reproductive age, making it more prevalent than thrush or UTIs.^³ It often goes unnoticed until it becomes recurrent or leads to complications. Beyond discomfort, BV is associated with increased susceptibility to sexually transmitted infections, and there is evidence linking BV-associated bacteria to pelvic inflammatory disease, which can damage the reproductive tract and affect long-term fertility.^³

Fertility and conception

Your vaginal microbiome plays a role in fertility that most women are never told about. Research now shows that a Lactobacillus-dominated microbiome - particularly one with high levels of L. crispatus - is associated with better IVF outcomes.^⁴ Women with vaginal dysbiosis have lower embryo implantation rates.^⁵ The overgrowth of bacteria like Gardnerella, Atopobium, and Prevotella is strongly connected with IVF failure.^⁶ This is not a peripheral concern for women undergoing fertility treatment. It is a clinically meaningful variable that is rarely discussed.

Pregnancy outcomes

Perhaps the most profound evidence of the vaginal microbiome’s importance comes from research on pregnancy. Multiple large-scale studies have established a clear relationship between vaginal dysbiosis and preterm birth - the leading cause of infant mortality worldwide.

Women with low levels of L. crispatus and elevated levels of BV-associated bacteria have significantly higher rates of spontaneous preterm labour.^⁷˒⁸ Pregnant women with dysbiosis before 20 weeks of gestation have been found to be at greater risk of preterm birth before 34 weeks.^⁹ A 2023 meta-analysis of over 2,000 pregnant women confirmed that the vaginal microbiome is one of the strongest predictors of early preterm birth.^¹⁰

What menstruation does to the microbiome

Every month, menstruation creates a set of conditions that the vaginal microbiome must navigate. Menstrual blood is alkaline...it raises vaginal pH and provides nutrients – such as iron – that dysbiosis-associated bacteria thrive on.^¹¹ Oestrogen levels fall, reducing the glycogen supply that feeds protective Lactobacillus populations. The result: vaginas that are Lactobacillus-dominated during one phase of the cycle may shift towards dysbiosis during and after menstruation.

This monthly vulnerability is not a flaw. It is simply biology. But it does mean that menstruation represents a recurring window - every 28 days (in a typical cycle), for 30 to 40 years of a woman’s reproductive life - during which the vaginal microbiome is at its at heightened vulnerability.

That is a significant amount of time. And it is precisely the window that period products occupy.

The product problem and the question nobody asked

The first commercial tampon entered the market in 1933. In the decades since, tampon design has changed remarkably little. The materials have shifted...from all cotton to cotton-rayon blends, from cardboard to plastic applicators. Synthetic polymers such as polypropylene and polyethylene are now widely used. But the fundamental premise remains the same: absorption. Nothing more.

What has changed, dramatically, is the science. We now understand that the vaginal microbiome is a living system with specific vulnerabilities. We know that menstruation is a recurring moment of disruption. We know that the materials and chemicals present in conventional tampons - synthetic fibres, chemical residues and processing additives can compound that disruption.

And yet: in the UK and EU, tampons are not required to be regulated as medical devices. They are classified alongside consumer goods...held to the same regulatory standards as toilet paper. Something that is placed inside the body, against sensitive mucosal tissue, for hours at a time, sits outside the framework that governs what can safely go inside the body.^¹²

The question that was never asked, across 90 years of tampon design, is the question that shapes everything we are building at The Daughters of Mars: what if the product that enters the body at its most vulnerable moment was designed to actively support it?

The five microbiome types and why yours matters

The Five Community State Types VAGINAL MICROBIOME ARCHITECTURES CST 1 L. crispatus Gold standard CST 2 L. gasseri Protective CST 3 L. iners Volatile CST 4 Polymicrobial High risk CST 5 L. jensenii Protective MORE PROTECTIVE HIGHER RISK Based on Ravel et al., PNAS 2011. Illustrations are representative, not microscopic. THE DAUGHTERS OF MARS

Here is something most women are never told: not all vaginal microbiomes are the same. Research analysing thousands of samples has identified five distinct configurations, called community state types or CSTs - and which one you have shapes your level of protection, your vulnerability to infection, and how your microbiome responds to disruptions like menstruation.² Think of your CST not as a diagnosis, but as a baseline: where your ecosystem starts from and how much it can absorb before it tips.

CST I — L. crispatus dominant. This is the gold standard. L. crispatus produces both types of protective lactic acid, keeps vaginal pH robustly acidic, and creates the most stable, resilient microbiome architecture we know of. Women with CST I have the strongest protection against BV, STIs and pregnancy complications.

CST II — L. gasseri dominant. Another healthy, protective type - slightly less robust than CST I but still associated with strong defence against infection and good reproductive outcomes.

CST III — L. iners dominant. This is where it gets interesting. L. iners is the most common species in the vaginal microbiome, found in roughly 20–40% of women and it is the most unpredictable.¹¹ Unlike L. crispatus, it can coexist with both protective and disruptive bacteria and it tends to shift under pressure. Hormonal changes, sex, antibiotics, certain products - all of these can push a CST III microbiome towards dysbiosis more readily than the others. Women with CST III are not unhealthy, but they are more vulnerable at transition points, including menstruation.

CST IV — low Lactobacillus, high diversity. This is the type most associated with bacterial vaginosis and elevated infection risk. Protective bacteria have been displaced by a mixed community, which is harder to stabilise. Importantly, not all CST IV microbiomes are symptomatic (some women are CST IV without knowing it) which is part of why BV is so chronically underdiagnosed. Lifestyle factors such as smoking significantly increase the likelihood of transitioning to CST IV.¹³

CST V — L. jensenii dominant. Rare (present in fewer than 10% of women) but highly protective. L. jensenii produces D-lactic acid and antimicrobial compounds, and is associated with very low infection risk and strong reproductive outcomes.

Your CST is not fixed. It changes across your cycle, across your lifetime and in response to what you expose your body to. That dynamism is part of what makes the vaginal microbiome so fascinating and so relevant to the products that can affect it. A tampon used during the most vulnerable phase of your cycle is not a neutral object. It is either working with your biology, or it isn't.

What disrupts the balance and what is within our control

The vaginal microbiome is resilient, but it is not indestructible. Many things that are simply part of daily life can shift its balance; sex, stress, diet, antibiotics, hormonal contraception, synthetic period products.^¹⁴˒¹⁵

Some of those disruptions are unavoidable. Antibiotics save lives but wipe out protective bacteria indiscriminately which is why BV recurrence rates remain so high even after successful treatment. An estimated an estimated 58% of women experience BV recurrence within 12 months of antibiotic treatment, in part because antibiotics address the symptoms without restoring microbial balance.^¹⁶

Other disruptions are designed in. Synthetic fibres. Chemical additives. Pesticides residues from cotton agriculture. Phthalates. Bisphenols such as BPA. PFAS (or ‘forever chemicals’). Chlorine-based bleaching processes. Products have rarely been evulated for their effects on vaginal health because the question was never on the agenda.

This is not about assigning blame to the women who use conventional products - there has been no meaningful alternative. It is about naming what the science says and asking what period care could and quite frankly should look like if it were built with this knowledge from the start.

Why this moment matters

The science of the vaginal microbiome has moved fast in the last decade. The ISALA project, a citizen science study of 3,345 women in Belgium, published in Nature Microbiology in 2023 - produced the most detailed map yet of vaginal microbiome composition and the factors that influence it. Menstrual product choice was one of them.^¹⁷

Research into the microbiome-pregnancy connection, the microbiome-fertility connection, and the mechanisms of BV is accelerating. Global experts like Dr Phillip Hay (one of the world’s leading researchers in bacterial vaginosis) are now turning their attention to how product design can either support or compromise this ecosystem.

And the regulatory conversation is starting slowly to catch up. There is growing pressure on manufacturers and governments to take the internal safety of period products seriously - not as consumer goods, but as products that enter the body.

The window for change is open. What has been missing, for 90 years, is the will to walk through it.

Want to be first to know when we launch? Join the founding waitlist for science led period care that works with your microbiome.

References

1. UN Women / Wellcome Trust data on reproductive health research funding.

2. Ravel J, et al. Vaginal microbiome of reproductive-age women. PNAS. 2011.

3. Vodstrcil LA, et al. Bacterial vaginosis: drivers of recurrence and partner treatment. BMC Medicine. 2021.

4. Lledó B, et al. Identification of vaginal microbiome associated with IVF pregnancy. Scientific Reports. 2022.

5. Balla B, et al. The Role of the Vaginal and Endometrial Microbiomes in Infertility. Int J Mol Sci. 2024.

6. Kong Y, et al. The Disordered Vaginal Microbiota Is a Potential Indicator for a Higher Failure of IVF. Frontiers in Medicine. 2020.

7. Fettweis JM, et al. The vaginal microbiome and preterm birth. Nature Medicine. 2019.

8. Gudnadottir U, et al. The vaginal microbiome and the risk of preterm birth: systematic review and network meta-analysis. Scientific Reports. 2022.

9. Ferrante M, et al. The vaginal microbiota and preterm birth: A systematic review. Eur J Obstet Gynecol Reprod Biol. 2025.

10. Callahan BJ, et al. Meta-analysis reveals the vaginal microbiome is a better predictor of earlier than later preterm birth. BMC Biology. 2023.

11. Brooks JP, et al. Changes in vaginal community state types reflect major shifts over time. mBio. 2017.

12. UK Medicines and Healthcare products Regulatory Agency (MHRA). Classification of medical devices. Current regulatory framework for period products.

13. Brotman RM, et al. Association between cigarette smoking and the vaginal microbiota. BMC Infectious Diseases. 2014.

14. Achilles SL, et al. Impact of contraceptive initiation on vaginal microbiota. Am J Obstet Gynecol. 2018.

15. Brown BP, et al. Copper intrauterine device increases vaginal inflammatory anaerobes and depletes lactobacilli. Nature Communications. 2023.

16. Bradshaw CS, et al. High recurrence rates of BV after oral metronidazole therapy. J Infect Dis. 2006.

17. Isala citizen science consortium. A citizen-science-enabled catalogue of the vaginal microbiome and associated factors. Nature Microbiology. 2023.