Where Does the Blood Go in Period Swimwear?

Where does the blood go in period swimwear - woman swimming confidently in outdoor pool - Savvi Wear

The blood goes into the absorbent core of the built-in gusset, a multi-layer panel sewn into the crotch of the swimwear, where it is trapped and held away from the water. The outer layer simultaneously repels pool and ocean water. What goes in stays in, and what's outside stays outside.

Key Takeaways

  • Menstrual flow is drawn into the absorbent core of the gusset and locked in. Research on knitted period underwear fabrics confirms bamboo yarn delivers superior fluid absorption and retention compared to other tested fibre types.
  • The waterproof TPU barrier layer keeps pool and ocean water out. Peer-reviewed TPU membrane research shows waterproofing raises the water contact angle from 83° to 105° - creating a reliable liquid barrier under pressure.
  • Inner layers are hydrophilic (they attract fluid inward); the outer layer is hydrophobic (it repels water). Absorbent product design research confirms this opposing three-layer architecture as the established standard for menstrual fluid capture and retention.
  • Savvi Wear's SW-1 Period Swimwear Bottoms hold 10–15 mL (2–3 tampons' worth). The first study to test this with real blood, in BMJ Sexual & Reproductive Health (2024), confirmed that honest capacity claims matter more than marketing figures.

How the Gusset Works

Period swimwear has a three-layer gusset, each layer doing a distinct job. The innermost layer is hydrophilic, it pulls fluid away from your skin inward. In Savvi Wear's SavviWear™ gusset, this is bamboo charcoal: its micro-gap cross-section gives it superior moisture absorption plus natural odour control. The middle layer is the absorbent core, where the blood goes and stays. The outermost layer is the waterproof TPU barrier: TPU creates a fully impermeable liquid barrier while remaining flexible and skin-safe, keeping absorbed fluid in and water out throughout your swim.

Does It Stay Put in the Water?

Yes, when the fit is correct and the flow is within capacity. Clinical menstrual flow data puts moderate daily flow at 6–10 mL, well within the 10–15 mL capacity of the Savvi Wear SW-1 gusset. The TPU barrier's low surface energy means water outside cannot reach the absorbent core, and absorbed fluid cannot escape back through it.

Frequently Asked Questions

Does the blood wash out in the pool?

No. The TPU barrier keeps absorbed fluid locked in the core throughout your swim. Afterwards, rinse immediately in cold water, hot water sets blood into fabric permanently.

Can anyone see or smell anything in the water?

No. The waterproof barrier contains all absorbed fluid within the gusset, and the bamboo charcoal lining provides natural odour resistance, confirmed in peer-reviewed healthcare textile research.

What if my flow is heavy?

On genuinely heavy days, pair period swimwear with a menstrual cup or disc for full coverage. Clinical guidance on menstrual products confirms period swimwear can be worn alone or combined for added protection. For light to moderate flow, standalone works well.

Is it safe and PFAS-free?

Savvi Wear's bamboo charcoal fabric is independently tested by SGS for 630 PFAS compounds, all non-detected. Research in Environmental Science & Technology (ACS, 2023) found PFAS at up to 64.6 ng/g across some hygiene product categories, making PFAS-free certification a meaningful differentiator.

Conclusion

The blood goes into the gusset's absorbent core and stays there, pulled inward by hydrophilic fibres, held by the bamboo charcoal core, and sealed in by the waterproof TPU barrier. For a deeper look at the full technology and how to choose the right style for your flow, read the complete guide to how period swimwear works or shop the SW-1 Mid Waist Period Swimwear Bottoms ($24.99) and see the gusset engineering for yourself.

Research References

  1. Study on the Permeability and Absorption Performance of the Crotch Layer in Seamless Knitted Period Underwear. Materials (MDPI), Vol. 17, No. 5 (2024). Found bamboo yarn superior to modal and viscose alternatives in fluid absorption and retention across 12 tested knitted fabric samples.
  2. Red blood cell capacity of modern menstrual products: considerations for assessing heavy menstrual bleeding. BMJ Sexual & Reproductive Health, Vol. 50, No. 1 (2024). First study to test menstrual product absorbency using real human blood; found real-world capacity differs meaningfully from water-based lab testing across 21 products.
  3. Physiology, Menstrual Cycle. StatPearls, NCBI Bookshelf (continuously updated). Clinical reference defining moderate menstrual flow as 30–50 mL per cycle (6–10 mL per day) and heavy flow as exceeding 80 mL per cycle.
  4. Hydrophobic Modification of Thermoplastic Polyurethane for Application in Waterproof and Moisture-Permeable Membranes. Materials (MDPI), Vol. 18, No. 21 (2025). Confirms TPU modification raises the water contact angle to 105°, significantly reducing water absorption and creating an effective waterproof barrier for textile applications.
  5. Per- and Polyfluoroalkyl Substances in Personal Hygiene Products: The Implications for Human Exposure and Emission to the Environment. Environmental Science & Technology (ACS), Vol. 57, pp. 8484–8495 (2023). Found PFAS present across multiple hygiene product categories at concentrations up to 64.6 ng/g — establishing the significance of PFAS-free testing in menstrual product manufacturing.
  6. Development of polyester-based bamboo charcoal and lyocell-blended union fabrics for healthcare and hygienic textiles. Journal of Industrial Textiles, Vol. 41 (2011). Confirmed bamboo charcoal increases antibacterial properties and moisture transmittance in textile applications relevant to hygiene products.
  7. Exploring biodegradable fibers as sustainable alternatives for sanitary napkins: A comprehensive review. Sustainable Materials and Technologies (Elsevier), Vol. 44 (2025). Reviews the established three-layer absorbent product architecture — permeable inner, absorbent core, waterproof barrier — underpinning modern menstrual product gusset design.
  8. TPU Coated Fabric Guide: Durability, Waterproofing, and Safety. EREZ Technical Textiles (2025). Technical reference on TPU waterproof fabric performance, biocompatibility, and application in hygiene and protective apparel.
  9. Bamboo Charcoal Yarns and Fabrics: A New Era in Textile Field. Fibre2Fashion (2009). Technical overview of bamboo charcoal yarn's micro-gap structure, superior moisture absorption, antimicrobial properties, and quick-drying performance.
  10. A pharmacist's role in increasing access to menstrual products: an education and advocacy approach. Journal of the American Pharmacists Association, PMC (2024). Clinical context noting period swimwear can be worn alone or combined with other products for added protection on heavier flow days.

About the Author

Stephanie Calleja is a marketing and content specialist at Savvi Wear who researches and writes on period health, swimwear, and sustainable care.

Important Disclaimer

Medical Disclaimer: This article is for educational purposes only. Savvi Wear products are designed to manage menstrual flow and are not medical devices. If you have concerns about your menstrual health, consult a qualified healthcare provider.

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