(CNN) — A period product that's eco-friendly, stylish and easy to use might sound impossible. But period underwear might be just that.

"Period underwear is a relatively newer period hygiene product that serves to be an alternative to more traditional products that people might be aware of, like pads and tampons," said Dr. Jessica Shim, a physician in the division of gynecology at Boston Children's Hospital and a clinical instructor at Harvard Medical School.

The technology used varies among brands, but typically involves several layers of material — such as cotton, nylon, merino wool, or polyester — geared toward absorbency of blood and wicking moisture from the vulva so the wearer feels dry, Shim said. Some brands use antimicrobial technology for combating odor or inhibiting bacterial growth.

Period underwear has grown popular over the past few years for several reasons. For one, being low-waste and reusable makes these products more eco-friendly than single-use ones. Conventional menstrual products can be up to 90 percent plastic, and about 49 billion of these single-use items are consumed annually in the United States alone, according to the United Nations Environment Programme's Life Cycle Initiative.

Brands behind conventional period products aren't required by the U.S. Food and Drug Administration to list every material included in their products, so knowing exactly what you're putting in contact with your body is another great reason to use period underwear, said Sarah Frank, a doctoral student and lecturer in the departments of sociology and legal studies at the University of Wisconsin-Madison.

Period underwear is also a good alternative for people getting comfortable with their changing bodies, those uncomfortable with inserting products into their bodies, athletes needing products to stay put and people who take care of menstrual hygiene for loved ones with disabilities, Shim said.

"Patients who are nonbinary [or] transgender may find it to be more convenient to use and not have to discard disposable products in public restrooms," she added.

Here's how to choose the best pair for you and care for them.

How absorbent is period underwear?

Period underwear brands offer a variety of absorbencies since people have different flows.

"They're typically saying the amount of tampons or sometimes the amount of blood that the underwear will absorb," Shim said. "The light one will absorb [the same amount as] one tampon and the medium absorbency underwear will list maybe two to five tampons."

Some brands recommend changing period underwear every 12 hours to prevent odor or leakage. Feeling wetness or skin irritation or seeing blood on your clothes can indicate that you need to change the pair because the underwear has absorbed all it can.

"Period underwear are designed to be a day-long or most-of-the-day option," said Dr. Kristyn Brandi, Darney-Landy Fellow of the American College of Obstetricians and Gynecologists, via email.

"If someone is bleeding beyond what the underwear can hold, it may be worth getting a different type or having an additional method like a tampon or menstrual cup."

How should period underwear be worn?

Period underwear can be worn on its own or for extra security while wearing a tampon, menstrual cup, or disc, Shim said.

If you're going to sleep in period underwear, make sure your pair's gusset runs long or high enough up your bottom so it can catch blood while you're lying down, Frank said.

How many pairs do I need?

How many you might need per day and for your whole cycle depends on your flow and how long your period typically lasts. You could need anywhere from a few to more than several pairs, Frank said.

Try wearing period underwear at home first so you can do a test run and make any needed adjustments in a safe space, Brandi said.

"For some people, they may be practical because it prevents trips to the store when you have run out of pads or tampons," said Dr. Jen Gunter, an OB/GYN and pain medicine physician based in California.

"For other people, changing underwear if you are out of the house and your period underwear is full might be impractical as you would need a backup and a plastic bag to take your bloody pair home," said Gunter, author of "The Vagina Bible: The Vulva and the Vagina — Separating the Myth From the Medicine."

What's the best way to wash period underwear?

Specific care instructions depend on the manufacturer's instructions, but typically, hand- or machine-washing period underwear with cold water is best, Shim said. These items can be washed alone or with other clothes.

If you hand-wash your underwear, do so until the water runs clear so you don't put the same blood and bacteria against your body again, Frank said.

Preserve the period underwear's technology and get more wears out of your supply by avoiding use of fabric softeners and by air-drying, Shim said.

If your pair has lots of absorbed blood, you can wring it out before throwing it in the washing machine, Frank said.

"It does require you to at least hands-on touch your underwear," Shim said. "For some people, that concept really is discomforting for them."

Do period underwear smell?

If you're not used to free bleeding, you might worry that you or others around you will be able to smell the blood sitting in an undergarment outside your body. But if you've properly cleaned your period underwear and haven't worn them past their absorbency capacity, there shouldn't be a noticeable smell, these experts told CNN.

What about the cost?

A box of around 40 tampons or pads can cost about $10 to $25, so seeing that one pair of period underwear costs about $15 to $50 can be intimidating, Frank and Shim said.

Over time, however, period underwear can be financially beneficial since they would replace the need to restock single-use products every month for the rest of the time you have a period, Shim said.

"Although it may be a long-term cost-effective option, it requires a sizable up-front investment, which means that it may not be an option that is equitably available to all people," Brandi said.

How long does period underwear last?

Period underwear can last for several years if properly cared for. But materials can wear down, so stocking up on several pairs isn't a one-time investment, Frank said.

Shifts in period flow and body size or shape are other factors that can influence how long you can wear the same pairs.

Does period underwear affect symptoms and period length?

Some people on social media platforms and product reviews have reported a lighter flow or shorter period while wearing period underwear. But experts say these observations are most likely more due to a difference in how much blood products absorb than the period underwear themselves changing someone's menstruation amount or length.

"[When] wearing tampons, for example, you're constantly thinking about when you need to change it," Shim said. "That might cause you to perceive your period to maybe be more bothersome and heavy because of how often you're changing out these disposable products, versus seeing a pair of underwear last you the whole day. That might give you a good sign like, 'Oh, OK, great — maybe my period is a lot more manageable than I've thought otherwise.'"

Anecdotal reports of milder cramps, however, could have more merit.

"I have not seen any data that would suggest that period underwear can alleviate symptoms, but not having a product inserted into the body could potentially impact the patient's experience of cramping," Brandi said.

Some people have pelvic floor muscle spasms and might feel pain using internal period products, Gunter said.

"People should be able to access and use whatever type of period management tools that work best for them, including period underwear," Brandi said.

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