It is a whisper passed down in locker rooms and poolside changing tents for generations. A young girl, nervous about a pool party falling water pressure on the wrong week of the month, is reassured by an older cousin or friend: “Don’t worry. The water stops it. You don’t even need to wear anything.”
For decades, this advice has served as a comforting shield against the anxiety of summer swimming. And for many, it seems to hold true—until it doesn’t.
The idea that water acts as a magical cork that pauses menstruation is one of the most persistent myths in female anatomy. Like most myths, it is rooted in a grain of scientific truth, but relying on it is a game of Russian Roulette played with physics. To understand why we bleed—and why we sometimes don’t—while submerged, we have to look at the battle between two fundamental forces: Gravity and Hydrostatic Pressure.
The Upward Push
When you stand on dry land, your menstrual flow is governed by gravity. The uterus sheds its lining, and gravity pulls that fluid down and out of the body. It is a simple, vertical equation.
However, when you step into a pool, you enter a different physical environment. Water is denser than air. Because of this density, it exerts force on any object submerged in it. This is called Hydrostatic Pressure.
This pressure pushes against your body from all directions—including upwards against the vaginal opening. Because the vagina is a collapsed tube (not an open pipe like a straw), this external water pressure can temporarily create a counter-force that exceeds the force of gravity.
Essentially, the water is pushing “up” harder than the blood is flowing “down.” As long as you remain relatively still and fully submerged, the flow may indeed seem to “stop.” The blood stays inside the vagina, held there by the weight of the water.
The “Sneeze” Factor
The problem arises because humans are not static objects. We move. We swim. We laugh. And we sneeze.
The balance of hydrostatic pressure is delicate. It relies on the internal pressure of your body remaining lower than the external pressure of the water.
However, if you engage your core muscles—by doing a flip turn, kicking hard to tread water, coughing, or sneezing—you create Intra-Abdominal Pressure. This internal force acts like a piston. It pushes down on the uterus and the bladder.
If this internal push is strong enough, it breaks the stalemate. The force of the “push” overcomes the resistance of the water, and a small amount of fluid escapes. This is why the myth is so dangerous for active swimmers. The very act of swimming (engaging the core) is the thing most likely to break the “seal” that the water provides.
The “Gravity Gap”
The second and most perilous moment occurs during the exit.
The “water stops it” theory relies 100% on being in the water. The second you begin to climb the pool ladder or walk up the beach, the hydrostatic pressure vanishes instantly. However, the blood that was pooled inside the vaginal canal during your swim is still there.
As soon as gravity takes over, that accumulated fluid releases. This is known as the “Gravity Gap”—the three to four seconds between leaving the buoyancy of the water and reaching for a towel. This is the moment where 90% of accidents happen. The false security of the swim transforms into a frantic rush to cover up, leading to the familiar “towel dance” or the anxious backward glance at the pool deck.
The Hygiene Equation
Beyond the mechanics of leakage, there is the issue of hygiene—not just for the pool, but for the swimmer.
While the water pressure provides some resistance to flow out, the exchange is not a one-way street. During menstruation, the cervix is slightly more open to allow the uterine lining to pass.
Swimming in public pools, lakes, or oceans without a barrier introduces the risk of “backwash.” Chlorinated pool water, while treated, is not sterile. It contains chemical byproducts and bacteria. Lake water contains algae and microbes. If you rely solely on water pressure without protection, you are exposing the vaginal microbiome to pH-disrupting elements that can lead to irritation or infections like Bacterial Vaginosis (BV).
The Modern Solution
For decades, the only answer to this physics problem was the tampon. It plugged the gap, absorbing the flow before it could challenge the water pressure. But for those who find internal protection uncomfortable, or for young swimmers just starting their cycles, the “tampon or nothing” binary was exclusionary. It led to many women simply sitting on the sidelines, “sunbathing” while their friends swam, feigning a dislike for the cold water to hide their biological reality.
Understanding the physics of hydrostatic pressure allows us to stop fearing the water and start preparing for it. We now know that the “seal” is temporary and fragile. It is not a magic force field; it is just a pressure differential waiting to be broken by a cough or a kick.
This knowledge validates the need for backup. It explains why modern textile innovations, such as period swim bottoms, have become such a vital tool in the swimmer’s kit. By recognizing that gravity always wins in the end—especially on the ladder—we can equip ourselves with gear designed to handle the “Gravity Gap,” ensuring that the only thing we leave in the pool is a ripple.
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