An Open Letter to Doctors, Gynaecologists and Women’s Health Professionals

Reconsidering the Safety of Primary Absorbent ("Bleed-Into") Period Underwear

Primary absorbent ("bleed-into") period underwear is increasingly being recommended by healthcare professionals as a safe alternative to traditional menstrual products.
However, after reviewing the available scientific literature, I have been unable to find long-term clinical or microbiological evidence demonstrating the safety of prolonged primary absorbent use under real-world conditions.(Krause et al.)

At the same time, several aspects of female reproductive physiology are already well established.
Healthy vaginal pH is typically acidic (approximately 3.5–4.5) and dominated by Lactobacillus species.(Amabebe & Anumba)

Menstrual fluid is a biologically active mixture of blood, endometrial tissue, mucus and vaginal secretions—not simply blood, and certainly not water.(Tindal et al.)

Menstrual fluid itself is near neutral to slightly alkaline (approximately pH 7.4), and menstruation is associated with a temporary rise in vaginal pH, reduced Lactobacillus dominance and increased bacterial diversity.(Chen et al.)

Menstrual fluid is biologically active, not just thin blood or water.

Vulvar tissue is more permeable and more susceptible to prolonged moisture, friction and occlusion than ordinary skin.(Farage & Maibach)

Recent blood-based absorbency testing has also raised questions about whether commonly advertised absorbency claims accurately reflect performance with menstrual blood.(DeLoughery et al.)

Taken individually, none of these findings demonstrates that prolonged primary absorbent underwear is harmful.
Taken together, however, they raise an important scientific question:

Has prolonged primary absorbent ("bleed-into") period underwear been shown to be safe, or have we begun reassuring women before the necessary evidence exists?

This letter explains why I believe that question deserves closer scientific scrutiny.

Background and Context

My name is Deepa Vari (formerly Deepa Kumar). In 2009, I invented India's first sanitary undergarment, later receiving patents in both India and the United States for the design. The product was created with one very specific purpose: to protect women and girls from menstrual leaks and stains while a pad, tampon, menstrual cup or other menstrual product performed the primary task of absorbing menstrual flow. It was never intended to become the primary absorbent product itself.

Over the past decade, products that outwardly resemble sanitary undergarments have evolved into something fundamentally different. Many are now marketed as primary absorbent menstrual products, encouraging women and girls to bleed directly into multilayer textiles for extended periods, often with claims of replacing several pads or tampons while remaining "dry," "fresh" and suitable for all-day wear.

That represents a fundamentally different method of menstrual management from the product I originally designed. Yet much of the public discussion continues to treat these products as though they belong to a single category. I believe that distinction matters.

Emerging Evidence and Physiological Mechanisms

Recent laboratory research using human red blood cells has reported substantially lower blood absorbency in “bleed-into” period underwear than commonly advertised capacities. (DeLoughery et al.). Although this study does not establish clinical safety or real-world performance, it raises legitimate questions about whether absorbency claims based on alternative laboratory methods accurately reflect how these products perform with menstrual blood.

Blood-based testing raises questions about absorbency claims made using water or saline.(DeLoughery et al.)

More importantly, several aspects of female reproductive physiology are already well established. Outside menstruation, the healthy vaginal environment is typically acidic (approximately pH 3.5–4.5) and dominated by Lactobacillus species that help protect against pathogenic microorganisms.(Amabebe & Anumba). 

During menstruation, the vaginal environment undergoes well-recognized physiological changes. Menstrual fluid is near neutral to slightly alkaline (approximately pH 7.4), and its presence temporarily raises vaginal pH.(Chen et al.).

During this phase, Lactobacillus dominance commonly decreases, bacterial diversity increases, and anaerobic organisms associated with bacterial vaginosis become more prevalent.(Chen et al.). 

These physiological changes are well described in the scientific literature and underscore that menstruation is a time of altered vaginal ecology rather than a simple extension of the non-menstrual state.

At the same time, vulvar tissue differs from ordinary skin. It is more hydrated, more permeable and more susceptible to prolonged moisture, friction and occlusion, making extended contact between absorbent textiles, retained menstrual fluid and the vulvar region a clinically relevant issue rather than a purely comfort consideration.(Farage & Maibach)

The Evidence Gap

What happens when women are encouraged to retain biologically active menstrual fluid within multilayer absorbent textiles that remain in prolonged contact with the vulvar region for eight to twelve hours at a time, month after month, year after year?

To my knowledge, we do not yet have long-term clinical studies answering that question.
We do not have dedicated microbiological studies examining the effects of prolonged primary absorbent underwear use. (Krause et al.).

We do not have long-term outcome data in adolescents who may wear these products throughout an entire school day.
We do not have evidence comparing prolonged primary absorbent underwear use with menstrual products that are routinely removed and replaced throughout the day. (Krause et al.)

In other words, the specific question that patients are increasingly asking has not yet been comprehensively investigated.

Scientific Position and Clinical Communication

I fully acknowledge that there is currently no conclusive evidence demonstrating that prolonged primary absorbent underwear use causes harm.
However, there is equally no conclusive evidence demonstrating that prolonged primary absorbent use is safe.
These are two very different scientific positions.

As clinicians, researchers and patient educators, we are rightly cautious whenever long-term safety has not yet been established. In most areas of medicine, uncertainty leads us to communicate caution rather than reassurance.

When established biological mechanisms raise reasonable scientific questions, and when the specific long-term studies needed to answer those questions have not yet been performed, is it appropriate to reassure millions of women and adolescent girls that prolonged bleed-into wear is broadly safe?

I believe that question deserves careful consideration. My intention is not to discourage innovation or to suggest that these products have been proven harmful. Rather, I believe women deserve complete transparency about the current state of evidence. They deserve to know which questions have been answered, which questions remain unanswered, and where scientific uncertainty still exists.

Suggested Approach for Clinical Guidance

Accordingly, I respectfully suggest three principles that could guide clinical and patient education about period underwear:

1. Clearly distinguish between sanitary undergarments designed as backup leak protection and underwear intended to function as the primary menstrual absorbent product.
2. Explicitly acknowledge that the long-term clinical and microbiological safety of prolonged primary absorbent wear has not yet been established through dedicated research. (Krause et al.).
3. Avoid language that could reasonably be interpreted as assuring patients that prolonged bleed-into wear has already been demonstrated to be broadly safe when that specific conclusion has not yet been established by the available evidence.

Finally, I encourage the use of terminology that better differentiates these products according to their intended function. Greater precision in language would help clinicians, researchers, regulators and patients discuss products with fundamentally different designs, patterns of use and evidence bases.

A Call for Responsible Inquiry

Medical progress has often begun by questioning assumptions before definitive evidence became available. Asking whether a product intended to improve women's lives could also carry unintended consequences is not opposition to innovation; it is part of responsible scientific inquiry.

Because millions of women and girls trust clinicians and leading institutions for menstrual health guidance, they deserve recommendations that reflect not only what science has established, but also what science has yet to determine.

References

1. Chen, C. et al. "Vaginal microecological characteristics of women in different physiological and pathological periods." Frontiers in Cellular and Infection Microbiology, 2022. Read the study →

2. DeLoughery, E., Colwill, A.C., Edelman, A. & Samuelson Bannow, B. "Red blood cell capacity of modern menstrual products: considerations for assessing heavy menstrual bleeding." BMJ Sexual & Reproductive Health, 2024;50(1):21–26. Absorbency comparison of 21 menstrual hygiene products using expired human packed red blood cells, finding period underwear absorbed less blood (roughly 1–3 mL) than advertised, versus other product categories. Read the study →

3. Farage, M.A. & Maibach, H.I. "Vulvar susceptibility to contact irritants and allergens: a review." Archives of Gynecology and Obstetrics, 2005. Read the study →

4. Amabebe, E. & Anumba, D.O.C. "The Vaginal Microenvironment: The Physiologic Role of Lactobacilli." Frontiers in Medicine, 2018. Read the study → 

5. Krause, A., Zane, G.K., Rivas-Nieto, A.C., et al. "Assessing the relationship between menstrual products and reproductive and urogenital tract infections (RUTIs): A systematic review evaluating the evidence and recommendations for future research." PLOS ONE, 2025. Read the study →

6. Tindal, K., Filby, C.E., Cousins, F.L., Ellery, S.J., Vollenhoven, B., Palmer, K., Gordon, A., Gargett, C.E. & Davies-Tuck, M. "The composition of menstrual fluid, its applications, and recent advances to understand the endometrial environment: a narrative review." F&S Reviews, 2024;5(3):100075. Read the study