Period Underwear: The Definitive Guide (Layers, Absorbency and Washing)

Period underwear is the fastest-growing category in period care in Spain, and that’s no accident: it’s regular-looking underwear with invisible absorbent technology that replaces (or backs up) pads and tampons. In this guide we explain how the layers work, how to choose the right absorbency and size, how to wash them so they last for years, and when to combine them with a menstrual cup. With concrete facts, no inflated promises.

What period underwear is and how it works

If you’ve made it this far, you’re probably considering the switch but a few reasonable doubts are holding you back: does it really absorb? Does it smell? How many pairs do I need? Is it worth the cost? Let’s answer them all, starting from the beginning.

On the outside, it looks like regular underwear. On the inside, a four-layer system stitched into the gusset absorbs flow, locks it in, and prevents leaks and odor. The difference from a pad is night and day: nothing shifts, nothing rubs, nothing crinkles, and at the end of the day you wash it and reuse it for years.

The top layer wicks flow toward the absorbent core and stays dry to the touch; the core holds the equivalent of 2-3 tampons depending on the style; a breathable but waterproof membrane stops leaks; and the outer fabric is cotton, just like your everyday underwear. With Sileu period underwear, the cotton is organic with OEKO-TEX Standard 100 certification — the international textile seal that guarantees fabrics free from harmful substances, which matters for something that’s in contact with your skin for hours.

Period underwear vs. pads: the honest comparison

The average person spends €5-10 a month on disposable pads or tampons. Period underwear at €9.99, which holds up through 2-3 years of washing, pays for itself within the first month or two. Add to that what you can’t measure in euros: sleeping without feeling like you’re “wearing something”, zero plastic in the trash, and zero disruptors in contact with your skin (more in our article on what pad and tampon marketing doesn’t tell you).

The weak point? With very heavy flow, underwear alone can fall short on the heaviest days. That’s where combining it with a cup comes in, which we cover below.

How to choose absorbency based on your flow

The practical rule: think about what you’d use that day if you were wearing tampons.

• Light flow or spotting days (start and end of your period, light spotting): underwear alone is more than enough.
• Medium flow: underwear holds up through a work day; it’s a good idea to change once you’re home.
• Heavy flow or long nights: underwear as backup for your menstrual cup: the cup collects the flow, and the underwear has you covered if there’s a small leak. It’s the ultimate leak-proof combo.

If you’re not using a cup yet and want to make the switch, our guide to choosing your first menstrual cup walks you through it; and if you’re coming from tampons, here’s how to make the switch without the stress.

How to choose your size

Go with your regular underwear size: period underwear should fit snugly (not tight) so the absorbent zone stays in contact with your body. If you’re between sizes, size down: loose underwear loses its leak protection. Sileu follows standard European sizing; you’ll find the exact measurements on the product page itself.

How many pairs do you really need

To use them as your main method, you need rotation: while some are washing and drying (air-drying takes a while), others are in use. A realistic count:

• As a cup backup or for light days: 2-3 pairs.
• As your main method: 4-6 pairs (2 a day on heavy days plus drying margin).

Starting with 2 and adding more once you see how they fit into your cycle is the smartest approach (and the one that risks the least money: €19.98).

How to wash them so they last for years

1. Rinse in cold water right after use, until the water runs clear. Cold, not hot: heat sets the stain.
2. Machine wash at 30°C with a mild detergent, ideally inside a wash bag.
3. No fabric softener, no bleach, no dryer, no iron: fabric softener coats the absorbent core (so it stops absorbing) and heat degrades the membrane.
4. Air dry, ideally laid flat and out of direct sun.

With this care, quality period underwear holds up for 2 to 3 years of regular use. If you notice it absorbing less or smelling after washing, it’s a sign it needs replacing (or that fabric softener snuck into the wash cycle).

For the full breakdown, with a step-by-step routine, hand-washing and troubleshooting for absorption or odor issues, we have a dedicated guide: how to wash period underwear so it lasts for years.

Period underwear for every stage of your cycle

One thing our customers tell us again and again: their underwear ends up getting used well beyond their period. Here are the scenarios where it shines most:

• Days before your period: if your cycle is irregular or you feel like it’s “about to start”, underwear removes the uncertainty without wasting a disposable “just in case”.
• Nights: sleeping in underwear (alone with light flow, or as backup with a cup on heavy days) is the difference between sleeping and sleeping soundly.
• Sports: it absorbs sweat and covers light exertion leaks; if you train with a cup in, it’s the perfect insurance (more in our guide to menstrual cups for athletes).
• Postpartum and spotting: for spotting between periods (we cover the causes in our article on spotting) and the postpartum weeks, when a cup isn’t an option yet.
• Light urinary leaks: the same absorbent technology covers the occasional leak from laughing or sneezing — if this happens often, you might also want to look into strengthening your pelvic floor.

What to check before buying any period underwear

The market has filled up with period underwear at every price point, and not all of it is equal. Four things worth checking, whatever the brand:

1. Fabric certification: make sure the fabric touching your skin is certified free from harmful substances (OEKO-TEX Standard 100 or equivalent). You’ll be wearing it for hours — this comes first.
2. Real composition: cotton on the inner layer beats synthetics; plastic-based fabrics breathe worse and are more likely to cause irritation.
3. Honest stated capacity: be wary of claims like “12 hours with any flow” without caveats. Real absorption depends on your flow, and brands that promise everything usually leave that part out.
4. Clear washing instructions: if the brand doesn’t explain how to wash it, it’s unlikely to last as long as promised.

Sileu underwear checks all four boxes, and the price (€9.99, or €18.49 for a 3-pack) is possible because we sell direct from the factory, no middlemen.

Underwear and menstrual cup: the leak-proof duo

Most of our long-time customers end up in the same place: cup inside, underwear outside. The cup collects flow for 8-12 hours, and the underwear removes the one remaining worry (what if something leaks while emptying it late, during sports, or overnight?). It’s also the favorite combo for the beach and the pool — underwear for before and after the water — and for postpartum, when the cup isn’t an option yet.