Population Variance in Pelvic Anatomy and the Case for Anatomy-First Menstrual Product Design

Nguyen Thi Huyen Duong
Founder & Lead Researcher
Smooth® & Cocmau®

September 2026

 

Reintroduction to the Female Body

Our brains are living in the future, our bodies in the past.

Medical textbooks did not feature the MRI imaging of the 3D structure of clitoris until 2005. Brands did not test menstrual products with real blood until 2023. Doctors have overprescribed oral contraceptives to women for six decades despite severe side effects such as mood swings, acne, and libido changes–the exact reasons why men dropped out of male hormonal contraceptive trials at advanced clinical trial stages. Endometriosis takes 8-10 years to diagnose. We have zero cure for menopause.

The race into the future has accelerated with such noble goals as AGI, self-driving transportation, robot marriages, and colonization of our neighbor planets. But there will not be anyone left to inhabit this future if we continue to ignore the biological reality of women of the here-now.

Women live their monthly periods with pains and uncomfortable products not designed for their bodies. Then they go to their doctors and get told that this is normal. They get on contraceptive pills for a decade before they build their family. By the time they stop, their bodies struggle to rebuild the hormonal pathway suppressed since menarche, which often leads to involuntary childlessness, fertility challenges, late parenthood, and endocrine disruptions.

Seven years ago, we decided to be a part of the future by solving a simple problem: making comfortable period products for Asian women. We made a reusable menstrual cup and a single-use biodegradable 12-hour disc. We did not expect to find our way into tens of thousands of ongoing, one-on-one conversations with women about their most acute pain to articulate.

Let me tell you what we found.

From Global Standard to Local Reality: The Bloody Gap

We have listened to thousands of Asian women complaining about their experience with Western-imported, traditional, bell-shaped menstrual cups that block the vaginal canal. “It pokes.” “It leaks constantly.” “It pains my abdomen.” “I can’t get it to open.” They either waste money testing through a series of unfit cups, or, after two cycles, they give up on the product that has the potential to change their lives – if it actually fits. 

In 2019, we deduced that we needed to make our Cocmau® Menstrual Cup shorter than the global average. Then we educated our users on the individual variations of cervical resting point measured from the introitus, and garnered close to 50,000 users via word of mouth in a few years, before hitting a local market ceiling, by which point we started to learn to run ads.

Before us, Big Period kept blaming Asian women for user errors. The women kept doubting their own bodies. We chose to believe the women, and made a business out of it.

In 2024, Stevens et al. came out with comparative multiplanar magnetic resonance imaging (MRI) studies and confirmed our initial hunch. The studies formally quantified the gap that Asian women experienced with their monthly products: The resting vaginal dimensions, wall lengths, and introital-to-cervical distances of ethnic Chinese and East Asian nulliparae range from 9% to 21% smaller than those of Western controls.

The problem must have lied elsewhere: a universal manufacturing standard led everyone in the supply chain to overlook this open secret. Someone designed the first cups for the Western/Caucasian baseline data, assuming an average undistended vaginal depth of 8.5 to 10.0 cm (Trowbridge et al., 2008; Pendergrass et al., 1996/2000). Then everyone copied this dimension without questioning the rationale behind it, and distributed it to the rest of the world. No one had touched this problem again since the filings of the patent for the first menstrual cup since the 1930s.

Validated by our own user metrics and Stevens et al.’s comparative MRI biometry, we started to hunt for more biometric data across Asia and were able to complete the bigger picture.

Scaling & Distribution of Pelvic Architecture: Biometric Proof

The 9-21% gap is a normal anatomical distribution. It is neither an anomaly nor a defect. Pelvic organ depths scale directly with skeletal frame and stature. 

Stevens et al. (2024) proved this natural gap with their MRI comparative analysis between East Asian populations vs. Western cohorts. They found a positive correlation between standing height and introital-to-cervical distance (r = 0.50).

Trowbridge et al. (2008) evaluated pelvic organ dimensions in community-dwelling Caucasian women using the standardized Pelvic Organ Prolapse Quantification (POP-Q) system. They gently extended a calibrated swab/depressor to the posterior fornix, recording an extended Total Vaginal Length (TVL) averaging 10.73 cm, ranging from 7.0 to 15.0 cm, with 25th to 75th percentiles spanning 8.5 cm to 11.0 cm and mean Point C located at -6.50 cm for an intact uterus. This aligns with broader Western multi-study syntheses where mean extended TVL consistently spans between 8.5 cm and 11.0 cm (Tan et al., 2006; Seo & Kim, 2006).

Pendergrass et al. (1996/2000) employed a direct anatomical method. With medical-grade vinyl polysiloxane casts, they measured the shape and volume of the vaginal vault in Caucasian cohorts, revealing a 6.9 cm to 14.8 cm distance between the vaginal opening and the cervical os, with population medians centering between 8.5 cm and 10.0 cm.

However, in 2006 Barnhart et al. evaluated the static, uncompressed depth of the vagina with landmark MRI biometry. They established a baseline of undistended resting vaginal length of 6.27 cm in healthy Western women.

By contrast, Seo & Kim concluded that the normative East Asian POP-Q data places average extended TVL at only 7.0 cm (2006). During active menses, the cervix descends into the vaginal vault (Flynn et al., 1988; Keefe, 1962; Nature Scientific Reports, 2025). For East Asian women, the average functional vaginal length falls within the [4.5 - 8.0] cm range when they need to resort to internal menstrual products (Seo & Kim, 2006).

Southeast Asian clinical research also validates this geometric compression. Extensive POP-Q evaluation of Southeast Asian cohorts by Lisa et al. (2014, n = 270) demonstrated that multiparous women experience a reduction in total vaginal length from 9.06 cm down to 8.65 cm, while the cervical position (Point C) descends significantly from -6.86 cm to -5.69 cm (p < 0.05).

In urogynecological device evaluations, a total vaginal length of < 7.5 cm is established as an independent predictor of device displacement and retention failure (OR = 2.7, p = 0.007) (Hanson et al., 2006). When this compact baseline is combined with the natural anatomical descent of the cervix during active menses (Flynn et al., 1988; Keefe, 1962), the functional depth available for internal collection devices frequently compresses to between 4.5 cm and 7.0 cm—well below the clearance required for standard 60–75 mm imported cups.

If East and Southeast Asian women attempt to insert a standard 70mm bell-shaped cup, the stem protrudes past the introitus, the rim pushes against their cervical os, the body applies excessive pressure to the mucosal tissues, and the cup refuses to fully expand or properly create a suction seal with the vaginal wall. This failed physics causes constant leaking and painful abdominal cramps.

Furthermore, demographic biometry cannot treat the Asian continent as an undifferentiated monolith. In a multi-centric prospective study of 1,562 Indian women across four tertiary centers, Mangla et al. (2025) established a mean Total Vaginal Length of 8.9 ± 1.4 cm. This demonstrates an intermediate biometric distribution between Western cohorts (10.7 cm extended) and the more compact 7.0 cm to 8.6 cm profiles observed in East and Southeast Asian populations (Seo & Kim, 2006; Lisa et al., 2014). 

These sub-regional variations reinforce our core thesis: medical device design must abandon coarse global templates in favor of population-specific anatomical calibration.

Diversity in Pelvic Architecture: The Parity Sizing Flaw

The 9-21% gap has direct implications for pelvic architecture, lifelong clinical outcomes, thus endovaginal product engineering.

In a systematic review, Mou et al. (2021) examined the prevalence of Pelvic Organ Prolapse (POP) across racial demographics by evaluating population-based screening data, which uncovered statistical disparities (p < 0.01):

Asian women have the lowest rate of pelvic organ prolapse – three times smaller than that of their Caucasian counterparts. A landmark study by Cheung et al. (2015) put this disparity down on a structural difference in pelvic floor biomechanics between ethnicities. Under maximal strain (Valsalva maneuver):

• Caucasian women experience a 32% expansion in their levator hiatus area.

• East Asian women experience a 19% expansion only.

Cheung et al. provided 3D/4D transperineal pelvic floor ultrasound (TPUS) data comparing the structural geometry and dynamic muscle behavior. By performing multivariate regression analysis controlling for age, height, weight, and BMI, the study found that ethnicity remained the single independent predictor of hiatal dimensions and organ mobility.

Biomechanical Interpretation

In Caucasian women, the tissue of the pelvic floor opening is more distensible (expanding by 32% under strain). The vaginal canal is also wider and deeper (by up to 21% compared to East Asian women), making it easier to accommodate larger menstrual cups. However, this higher tissue elasticity also makes the internal organs more vulnerable to downward placement later in life (a 10.76% prolapse rate).

In East Asian women, the resting vaginal canal is 9-21% smaller, the pelvic floor opening is lined by a thicker pubovisceral muscle, expanding by only 19% under strain. This compact anatomical structure provides mechanical support against prolapse. It is a biological strength, yet actively resists large, long, firm menstrual cups designed for an entirely different pelvic architecture.

The Flaw of Parity-Based Sizing

The biomechanics of the pelvis exposes another flaw in traditional period product design: sizing cups based on childbirth history.

After a woman has given birth vaginally, most period care brands recommend her to switch to a larger cup size. However, industry sizing algorithms scale both diameter AND height of the cup, simultaneously.

Anyone can spot the anatomical contradiction here. Childbirth may increase introital capacity, but it does not lengthen the vaginal vault. If anything, postpartum tissue laxity and mild pelvic descent cause the cervix to sit lower in the canal. As proven by Lisa et al., in the Indonesian cohorts, the average resting cervical position dropped from -6.86 cm to -5.69 cm, a descent of  ~1.1 cm post childbirth. 

It makes no sense to design a cup that is both wider and taller for a post-birth woman. If she tries to force such an object into an already shortened canal, she is guaranteed to experience major sizing failure.

The Ergonomic Solution: Population-Based Calibrated Design

A compact, high-tone pelvic architecture requires the period industry to move away from archaic global templates toward population-calibrated ergonomic engineering.

To serve bodies characterized by a 4.5 cm to 8.0 cm functional cervical height and low hiatal distensibility, product designers must deploy two distinct mechanical strategies:

Strategy 1: Shortened Bell-Shaped Cup Geometry (Cocmau®)

To accommodate a functional cervical height of 4.5 – 7 cm during active menses, the vertical height of a menstrual cup must be reduced from the archaic global standards:

1. Truncated Body Length: The body of the cup must be between 42 mm  and 52 mm, preventing the base or stem from protruding past the introitus and irritating sensitive mucosal tissue.

2. Flexible Ring Removal Mechanisms: Replacing long stems or short rigid stems with an ultra-soft ring pull mechanism eliminates introital poking while providing effortless reach and grip in a shorter canal.

3. Trimmable Stem/Ring Adjustability: The ability to shorten total device length (by cutting off the ring stem, leaving no sharp edges behind) accommodates for postpartum cervical descent without forcing the user to switch to a different, ill-fitting cup size.

4. Calibrated Rim Tension & Inverted Rim: Because East Asian pelvic floor muscles exhibit higher tone and lower distensibility (Cheung et al., 2015), the upper rim spring tension must be engineered to pop open smoothly without exerting excessive radial force that triggers reflex uterine or pelvic floor cramping. For Cocmau®, we chose an inverted rim mechanic to minimize this outward spring pressure.

5. Independent Width vs. Height Scaling: Postpartum cups must increase only in rim diameter to match introital tone, while keeping vertical body height short to avoid impinging on a lower cervical axis.

6. Eradicating Parity Based Sizing: Coarse “pre-birth vs. post-birth” sizing matrices reinforce stigma around natural childbirth but ignores pelvic depth. Educating consumers on actual pelvic biomechanics gives them true agency to select hardware that fits their bodies. It is time to part with generic empowerment gender rhetoric.

Strategy 2: The Fornix-Seated Anatomical Bypass (Smooth® 12-Hour Period Disc)

While a shortened cup with a removal ring stem solves the cervical height barrier for tens of thousands of users, an alternative architecture bypasses cervical height dependency altogether: the fornix-seated menstrual disc.

Bell-shaped cups sit at the vaginal canal and rely on a suction seal against the vaginal walls. Meanwhile, a menstrual disc is inserted horizontally into the vaginal vault, sitting deep inside the vaginal fornix, creating a thin barrier below the cervix, its rim wedging behind the pubic symphysis.

As a menstrual disc occupies the fornix, it operates at the widest part of the pelvic cavity, leaving the lower vaginal canal unobstructed. The deep, flexible basin collects menses directly at the cervical os regardless of its distance from the introitus, making the disc a universal fit whether a woman has a cervical height of 4.5 cm or 8.5 cm.

Bioethical Framework: Population Baselines vs. Precision Ergonomics

We must draw rigorous ethical boundaries when introducing demographic biometry into medical hardware. Physical distribution across ethnicities belongs to a normal category of population-based industrial design and medical research, distinct from biological determinism, racial essentialism, or the revival of archaic typologies.

Responsible hardware for the human body must obey three ethical pillars governing how biometric data is interpreted and applied:

Rejecting Biological Essentialism and Monolithic Assumptions

Human anatomical variations exist within a continuous spectrum, rather than within discrete, impermeable racial categories.

Statistical findings established by modern population genetics and bioethics – such as the population-level 9-21% reduction in resting vaginal canal depth (Stevens et al., 2024) or a 19% hiatal expansion limit under strain (Cheung et al., 2015) – represent centralized tendencies within sampled cohorts. These are not racial absolutes. Moreover, within every population there also exists substantial intra-group variation. There are East Asian women with high cervical positions requiring longer devices, just as there are Caucasian women with compact pelvic vaults or prolapsed cervixes experiencing pain with standard-sized cups.

A demographic baseline provides an engineering roadmap for product development. It must never dictate what an individual “needs” based on ethnicity alone, commercially or clinically speaking.

Dismantling the “Universal Western Default”

The Western body has been treated as the “universal human standard” in biomedical engineering, pharmacological testing, and consumer health design. Any deviation from this Western baseline was either statistical outliers or user errors. This negligence must change now.

Even “Asia” is not an anatomical monolith (see “Scaling & Distribution of Pelvic Architecture: Biometric Proof” above). South Asian women sit at an intermediate baseline (~8.9 cm) between Western cohorts (~10.7 cm extended) and East/Southeast Asian cohorts (7.0 cm to 8.6 cm). This gradient across global populations only proves the need for population-calibrated tooling.

True inclusivity in medical design means doing the empirical work to ensure that hardware accommodates the reality of human diversity. It should not mean pretending all bodies are identical. The period industry tried doing so, only to be met with market-level ergonomic failure while turning down several of the fastest growing consumer health markets.

Reframing Compact Architecture as Functional Strength

Medical literature committed the historical error of framing non-Western anatomical baselines through a deficit lens, labeling them as “underdeveloped” or “diminutive”.

The data from Mou et al. (2021) and Cheung et al. (2015) refutes this framing. The compact pelvic architecture and higher pubovisceral muscle tone characteristic of East Asian cohorts mean superior mechanical support, making them three times less vulnerable to pelvic organ prolapse compared to Western cohorts.

A compact pelvic floor is, thus, a biomechanical asset. Failure with insertion of endovaginal products has never been with Asian women’s anatomy or user errors. It is high time the period industry took full responsibility for how they manufactured hardware for high-distensibility pelvic floors and forced it onto high-tone, supportive structures.

From Demographic Sizing to Precision Ergonomics

The ultimate goal of demographic biometry is to move the entire industry toward precision ergonomics. We reject the commercial segregation of consumers by ethnicity.

Population-level data instructs manufacturers on how to allocate tooling capital, calibrate rim spring tensions, and diversify product portfolios to match the requirements of the market they serve.

At the point of care and purchase, however, hardware selections must be personalized.

Our research aims to give every woman the vocabulary, anatomical awareness, and product options to evaluate her own cervical height and pelvic mechanics, giving her the agency to choose hardware that respects her body.

We recognize that this principle sits in tension with our go-to-market language. Marketing a product line as “period care built for Asian anatomy” is a population-level claim, made to reach the most underserved segment by existing default. It is not, and should not be read as, a clinical instruction that every individual of this population needs these specific dimensions, nor that no one outside of it might. We hold both claims to be true: population-level design is a legitimate and necessary approach; product selection process should be anatomy-first, never identity-first.

Limitations and a Forward Validation Agenda

We limit the scope of this paper to a synthesis of independently published anatomical and biomechanical literature. It is neither a systematic review nor a clinical trial of our own products. Read it as a design rationale, supported by outside evidence. We are not trying to prove that SmoothDisc™ or Cocmau® outperform the alternatives in a controlled setting. Three limitations deserve explicit attention:

Our own outcome data: a preliminary sizing signal

To date, Cocmau® Menstrual Cup has recorded zero returns attributable to sizing or fit failure and zero drop-off rate after 3 menstrual cycles. This is our actual internal metric and not an estimate. We have not, however, disclosed a full reason-for-return taxonomy alongside this metric, nor have we discussed the usage time windows our product covers. We intend to do so in a future revision of this paper. Until then, these metrics should be read as directional company evidence, rather than an externally audited or verified figures.

A second signal comes from how the product acquired its earliest adopters in the first place. The vast majority of our initial users arrived, having already owned and abandoned a generic, Western-mold import cup — most commonly within two menstrual cycles — citing abdominal pressure, pain, leakage, and a stem that protruded and irritated the introitus. These are the same failure modes this paper attributes, mechanistically, to fitting a longer bell-shaped body and stem onto a shorter functional vaginal canal depth (see “This failed physics causes constant leaking and painful abdominal cramps” above). We present this as corroborating, not confirmatory, evidence: it is drawn from customer intake conversations, rather than a structured, instrumented survey, so it has no denominator, no comparison group, and no standardized reason-for-abandonment coding yet. It is consistent with the biomechanical argument and helps explain organic, word-of-mouth-adoption independent of marketing spend, but it is not, on its own, proof of the mechanism.

The evidence base is thinner outside East and Southeast Asian cohorts

The core anatomical studies we rely on (Stevens et al., Seo & Kim, Cheung et al.) are drawn from ethnic Chinese and Korean nulliparous populations. For the Southeast Asian populations, we make observations on Indonesian Pop-Q benchmark studies (Lisa et al., 2014). We have established that cervical height must always be evaluated relative to total vaginal length (C/TVL) rather than treated as a fixed universal measurement (Oh et al., 2023).

We have not established, and do not claim, that findings generalize to South Asian or perimenopausal and postmenopausal populations, where tissue elasticity and canal dimensions change independently of the mechanisms discussed here. Product claims outside the studied populations should be treated as a hypothesis carried over by analogy, not as established fact, until tested directly.

Engineering claims are not yet backed by published bench or usability data

The specific parameters we propose — a 42–52 mm truncated body length, inverted rim tension, ring-stem geometry — are engineering conclusions drawn from the anatomical literature, not results of a published fit trial, material fatigue study, or biodegradation test against a recognized standard (e.g., ASTM D6400 for the disc). We intend to publish this validation data in a future revision of this paper rather than assert performance claims the current evidence does not yet support.

Hardware for the Bleeding Bodies: A Conclusion

Humanity is accelerating toward AGI, autonomous systems, and space colonization. Yet the global period care industry got slow and lazy: it relied on one single patent from 1937 to force one generic baseline onto hundreds of millions of women (Chalmers, 1937). It dismissed customer feedback and did not even bother to connect the dots with pelvic diversity for nine decades. It is a great irony that the very industry so keen on calling out double standards and gender gaps forgot to fix the problem at its root.

A modern woman has more than 400 periods in her lifetime. That means 5 years spent on bleeding alone. Their pain, shame, and discomfort are valid and a matter of human continuity.

We have proven the “universal standard” wrong with empirical evidence gathered from MRI biometry, standardized POP-Q clinical trials, dynamic ultrasound biometry, and tens of thousands of real-world consultations:

1. Pelvic Depth Scales with Stature: Internal resting dimensions scale with skeletal frame and standing height (r = 0.50). East Asian populations have resting vaginal length and introital-to-cervical distances that are 9-21% smaller than Western averages, with functional depths during menses centering between 4.5 cm - 8 cm (Stevens et al., 2024; Seo & Kim, 2006)

2. Compact Architecture is Structural Asset: East Asian pelvic floors, with thicker pubovisceral muscles, expand by only 19% under strain, compared to 32% in Caucasian cohorts (Cheung et al., 2015). This compact architecture provides superior support that reduces the incidence of pelvic organ prolapse three-fold, while actively resisting long, rigid devices engineered for high-distensibility structures.

3. Parity Sizing is Mechanically Flawed: Childbirth does not lengthen the vaginal vault. Scaling menstrual cup height alongside diameter for post-partum bodies forces an unnaturally long device into a shortened canal where the cervix descends, guaranteeing fitting failure.

We have eliminated the problem simply by honoring human biometry: truncating the vertical profile of a bell-shaped cup to 42-52 mm, softening its radial tension, and eliminating the parity sizing matrices. Transitioning our product portfolio to include a fornix-seated disc removes users’ dependency on cervical positions and pelvic distensibility altogether.

True progress in women’s health will not come from corporate slogans dressed in gender discourse and empowerment rhetorics that conceal outdated manufacturing practice. True progress can only be downstream of first-principle engineering: taking an honest look at biological data, questioning and rejecting defaults, and designing hardware for the human body that bleeds.

This is the era in which biological hardware, just as software does, adapts to women’s biological reality.

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