Menstruation is the cyclical shedding of the uterine lining, resulting in the temporary discharge of blood and tissue. Many people who menstruate notice that when they enter a swimming pool, lake, or ocean, the external flow of menstrual fluid seems to pause entirely. This observation has led to the common but inaccurate belief that being submerged causes the menstrual cycle to stop. The internal biological process continues uninterrupted, but physical forces at work within the water create the illusion of flow cessation.
The Role of Water Pressure in Flow Reduction
The perceived stoppage of menstrual flow while swimming is a phenomenon explained by the principles of physics, specifically hydrostatic pressure. Hydrostatic pressure is the force exerted by a fluid at rest, and this pressure increases with the depth of the water. When the body is submerged, the surrounding water applies an external, uniform pressure against the entire body, including the vaginal opening.
This external force directly counteracts the internal forces that normally push menstrual fluid out of the body, primarily gravity and the rhythmic muscular contractions of the uterus. The pressure exerted by the water creates a temporary physical barrier, preventing the fluid from escaping and mixing with the surrounding water. The higher density of water compared to air provides enough resistance to overcome the relatively slow movement of menstrual flow.
The uterine lining continues to shed, and the menstrual fluid still collects within the vaginal canal while a person is submerged. The flow is not truly stopped, but rather minimized and held in place by the counteracting water pressure. If a person were to cough, sneeze, or laugh forcefully while submerged, the sudden internal pressure could briefly overcome the external hydrostatic pressure, potentially allowing a small amount of fluid to escape. This temporary pooling of fluid inside the body contributes to the noticeable gush of blood once the person leaves the water.
What Happens Immediately After Exiting the Water
The moment a person leaves the water, the hydrostatic pressure that was acting as an external barrier instantly vanishes. This sudden removal of the counteracting force immediately restores the normal dynamics of menstrual flow. Gravity again becomes the dominant external force, acting to pull the accumulated fluid downward and out of the body.
The fluid that had been collecting in the vaginal canal during the time spent submerged is then released rapidly, often giving the impression that the period has instantly resumed or “started again.” This gush is not a sudden increase in the rate of menstruation, but simply the discharge of the volume that had been temporarily retained. The internal contractions of the uterus also continue to push the fluid out, contributing to the perceived rush of flow.
The experience of a sudden, heavy flow upon exiting the pool is simply the body catching up on the discharge that was delayed by the physics of the water. This effect highlights that the menstrual process was ongoing, even when the external signs of flow were not visible. Preparing for this immediate resumption of flow is an important consideration for comfort and preventing leakage after swimming.
Managing Menstruation While Swimming
Since the internal process of menstruation continues while submerged, using appropriate period protection is recommended for comfort and hygiene. Products designed for internal use, such as tampons, menstrual cups, or discs, are the most effective options for managing flow during water activities. Tampons absorb the fluid before it exits the body, while menstrual cups and discs collect the fluid.
Menstrual cups are often a preferred choice because they create a seal that collects the fluid without absorbing any surrounding water, unlike tampons, which can become saturated with pool or ocean water. Conversely, external products like pads and panty liners are not suitable for swimming because their absorbent material will quickly soak up the surrounding water. This saturation renders them ineffective for absorbing menstrual fluid and can cause them to become bulky or detach.
Swimming during menstruation is completely safe and hygienic, even without internal products, as any minimal fluid release is quickly diluted in a large body of water. Pool water is also treated with chemicals like chlorine, which neutralize organic matter. Many find that the light exercise of swimming can help relieve symptoms such as menstrual cramps and bloating, making it a beneficial activity during their cycle.