Period pads absorb menstrual fluid through a layered system designed to pull liquid away from your body, lock it in place, and prevent leaks. Each layer has a specific job, from the soft surface against your skin to the waterproof barrier at the bottom. Understanding how these layers work together explains why some pads feel dry even after absorbing fluid, and helps you choose the right one for your flow.
The Four Layers Inside a Pad
A disposable pad is built from four distinct layers stacked on top of each other, each made from different materials.
The top sheet is what touches your skin. It’s made from perforated nonwoven polypropylene or polyethylene, both lightweight thermoplastics with tiny holes punched through them. These holes let fluid pass through quickly while the material itself repels moisture, so the surface stays relatively dry against your body. Many pads also have a thin emollient coating on this layer to reduce skin irritation from prolonged contact.
The distribution layer sits just below the top sheet. Its job is to spread incoming fluid across a wider area rather than letting it pool in one spot. This gives the absorbent core below more surface area to work with, which speeds up absorption and reduces the chance of overflow.
The absorbent core is where the real work happens. Modern pads use a blend of wood pulp cellulose and a superabsorbent polymer derived from acrylic acid and sodium hydroxide. This polymer can hold many times its own weight in liquid. When menstrual fluid reaches the core, the polymer chains attract water molecules and swell into a gel, trapping the fluid so it can’t squeeze back out under pressure. The cellulose fibers provide structure and help distribute fluid evenly through the core.
The back sheet is a thin waterproof film, typically polyethylene, that prevents any absorbed fluid from soaking through to your underwear. This layer also carries the adhesive strip that attaches the pad to your clothing.
How the Pad Pulls Fluid Away From Your Skin
The key to a pad feeling dry is wicking, the process of moving liquid from one layer to the next through capillary action. The top sheet’s perforated design creates small channels that draw fluid downward by surface tension. Because polypropylene and polyethylene are hydrophobic (they naturally repel water), fluid doesn’t linger on the surface. Instead, it’s pushed through the perforations and into the more absorbent layers beneath.
Once fluid reaches the distribution layer, it spreads laterally. This is important because menstrual flow doesn’t land evenly across the pad. Without this layer, you’d end up with a saturated center and dry edges, which leads to leaks. The distribution layer acts like a funnel in reverse, directing fluid outward so the entire absorbent core gets used.
Inside the core, the superabsorbent polymer converts liquid into gel almost immediately on contact. This gel form is stable, meaning it won’t release fluid even when you sit, move, or apply pressure. That’s why a used pad can feel heavy but not wet to the touch.
How Wings and Adhesive Prevent Leaks
The adhesive strip on the back sheet sticks the pad to the inside of your underwear so it doesn’t shift forward or backward as you move. Pads with wings add an extra layer of leak protection: the wings are flaps that fold over the edges of your underwear and stick to the outside, anchoring the pad more securely. This helps in two ways. It keeps the pad centered where it needs to be, and it creates a barrier along the sides of your underwear where fluid might otherwise escape.
Wings are especially useful during physical activity or overnight use, when your body shifts positions frequently. Without them, a pad can bunch or slide to one side, exposing gaps where leaks happen.
How Much Fluid Pads Actually Hold
Pad capacity varies significantly by size and brand. In lab testing published in BMJ Sexual & Reproductive Health, a light-day pad held about 4 mL of fluid, while heavy-day pads held between 31 and 52 mL depending on the brand. Postpartum pads, which are larger and thicker, held around 40 mL.
For context, total menstrual flow over an entire period averages about 30 to 40 mL for most people, though it can be much higher. The heaviest flow days typically happen in the first two to three days, which is when you’ll go through pads fastest. A regular pad doesn’t need to hold your entire period’s worth of fluid. It needs to handle a few hours of flow reliably, which is why changing pads regularly matters more than choosing the highest-capacity option.
How Often to Change Your Pad
The CDC recommends changing pads every few hours regardless of how light your flow seems, and more frequently on heavy days. Even if a pad doesn’t feel full, trapped moisture creates a warm, damp environment where bacteria and fungi can grow. Wearing the same pad too long can lead to skin rashes or infections.
Scented pads might seem like they’d help with freshness, but they can actually irritate skin and disrupt your natural pH balance. Unscented pads are the safer choice for prolonged wear. The odor associated with periods comes from fluid breaking down after exposure to air, not from the fluid itself, so changing pads regularly is more effective than fragrance at keeping things fresh.
What Pads Are Actually Made Of
Despite their soft, fabric-like feel, disposable pads are overwhelmingly plastic. A single pad can contain up to 90% plastic components, from the polypropylene top sheet to the polyethylene back sheet to the synthetic polymers in the absorbent core. The adhesive strips, wing material, and individual packaging add more.
This matters if environmental impact is a concern for you. Conventional pads don’t biodegrade in any meaningful timeframe. Reusable cloth pads, period underwear, and menstrual cups are alternatives that reduce waste significantly. Some disposable brands now use plant-based or biodegradable materials in place of conventional plastics, though these still vary in how well they perform compared to standard designs.
For most people, choosing a pad comes down to balancing absorbency, comfort, and how the materials feel against your skin. Knowing what’s inside each layer makes it easier to understand why some pads feel drier, thinner, or more flexible than others, and why the price range between basic and premium options can be so wide.