The observation that menstrual bleeding seems to cease when submerged in water is an intersection between physics and human physiology. This perceived stoppage is not a magical halt of the biological process, but a temporary effect caused by external forces acting on the body. The menstrual cycle continues its normal function internally, shedding the uterine lining whether you are on land or in the water. Understanding this phenomenon requires looking at the physical principle of pressure.
The Role of Water Pressure
The primary reason bleeding appears to stop is the basic principle known as hydrostatic pressure. When the body is submerged, the surrounding water exerts pressure evenly across the entire surface, including the vaginal opening. This external force acts as a counter-pressure to the outflow of menstrual fluid.
The pressure exerted by the water is greater than the slight internal pressure pushing the fluid out, effectively creating a temporary seal. This counter-force slows or prevents immediate leakage while you are submerged and relatively still. This hydrostatic force allows a person to swim without visible blood escaping the body.
However, this pressure can be easily overcome by internal forces like a cough, a sneeze, or a sudden change in position. In such cases, a small amount of blood may escape into the water before the external pressure reasserts itself.
What Happens to the Menstrual Flow Underwater
The body does not stop menstruating once it enters the water. The process of shedding the uterine lining continues, and the menstrual fluid simply collects temporarily inside the vaginal canal. The flow is significantly slowed and contained by the water pressure, but the internal biological action is uninterrupted.
Menstrual fluid is not just thin blood; it is a mixture of blood, tissue, and mucus, giving it a thicker consistency than water. This difference in viscosity means the fluid does not instantly mix or disperse upon contact with water. If a small amount does escape, the sheer volume of water quickly dilutes it, making it essentially invisible.
Gravity’s influence also changes when the body is submerged. On land, gravity pulls the fluid downward, encouraging flow. In water, the body’s position and buoyancy further inhibit the normal downward exit of the fluid. The flow typically resumes its normal rate immediately upon exiting the water, as the hydrostatic pressure is instantly removed.
Swimming Safely While Menstruating
Swimming is safe and hygienic during menstruation, and it can help relieve cramps through light exercise. The primary concern is managing the flow for comfort and to avoid leakage when exiting the water.
Internal menstrual products are the most effective solution for containing the flow. Tampons or menstrual cups are recommended because they collect the fluid internally and will not become saturated with water like external products. Pads and panty liners are not suitable for swimming because their absorbent material quickly soaks up water, rendering them ineffective.
A menstrual cup is a particularly good option because it creates a seal and collects the fluid without absorbing surrounding water. If using a tampon, change it immediately before and after swimming to maintain hygiene. Period swimwear, which features an absorbent, leak-proof layer, provides a discreet option for lighter flow days or as a backup.