Agrawal, 37, is the co-founder and chief executive of Thinx, a company that sells multi-layer, antimicrobial, leakproof panties. The self-described social entrepreneur is part of a growing wave of businesswomen who are harnessing new technology and changing attitudes to disrupt the $19 billion feminine hygiene market.

Eight decades after a male doctor patented the tampon in the United States, and more than a century after the first disposable pads landed in stores, these pharmacy staples still dominate the field, boasting generations of customer loyalty. Since 2014, though, Thinx and its cohort of women-led start-ups have tried to shake up the two-party system of period protection, introducing such products as moisture-wicking thongs and insertable, liquid-catching latex discs.

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These businesses think the market is ready for them, largely because people are openly talking about periods. Lawmakers across the country are slashing taxes on menstrual products, arguing that these are mandatory purchases for about half the population. A runner finished the 2015 London marathon in crimson-soaked leggings, telling interviewers that menstruation “does exist and we overcome it every day.” A Chinese Olympian in Rio de Janeiro complained about her period on television. A British model talked to reporters last month about how her cycle had thwarted casting calls.

The entrepreneurs are exploiting this conversation, reaching out to millennials with frank Facebook videos and tweets. They describe the realities of the female body like cool older sisters, touting their inventions as healthier or lower maintenance.

Norms passed down from mother to daughter, however, are tough to break.

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“We’re not supposed to keep pads or tampons on for too long,” a skeptic wrote recently on Thinx’s Facebook page. “I don’t think sitting in your menstrual blood for too long is sanitary.”

Retail forecasters see no imminent consumer exodus from the traditional disposables — pads, for example, accounted for 43 percent of the market’s revenue last year, by one firm’s count — but companies that sell alternative goods are independently reporting sales spikes, especially over the past two years.

The history of pads

The first commercial menstrual pad hit the nation in 1896, with Johnson & Johnson advertising sanitary napkins crafted of cotton. Almost four decades later, a Colorado doctor named Earle Haas secured the patent for what would become the modern tampon.

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Lauren Schulte, the 30-year-old creator of the Flex Company, hates both products. Tampons, she points out, require frequent changing to prevent leaks and toxic shock syndrome, a bacterial infection linked to prolonged use. Pads, she said, are just uncomfortable.

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Schulte envisioned a product that would allow her to live as though she were not bleeding. Would some kind of seal work? One you could insert and essentially forget? And wear during sex? She ran the idea by some entrepreneurial friends in 2014, hoping to inspire them to look into it.

“I never intended to be a founder,” said Schulte, who worked in marketing at the time. “They said, ‘Uh, Lauren, you have every capability to do this yourself.’ ”

Despite her reservations, she agreed. She poured $70,000 of savings into developing her dream product: the Flex, a puck-shaped seal made of flexible medical-grade polymer, which captures blood for up to 12 hours before the user throws it away. “Mess-free sex” is one of its selling points.

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Investors, she said, have loved it — so far, Schulte has raised $4 million. The Flex is registered with the Food and Drug Administration. The first shipment, offered as a free trial, hit 20,000 customers this month. A 24-pack costs $15 per month, if a buyer subscribes to the delivery service. A 36-pack of Tampax Pearl tampons at Walgreens, for comparison, costs less than $9, but the life span of a tampon is four to eight hours.

Schulte said the budding discussion concerning menstruation emboldened her to pursue her idea.

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“The more of us that go out and talk about our own experiences,” she said, “the more mainstream it becomes.”

She entered the business of period management as the face of entrepreneurialism changed. Over the past nine years, the number of woman-owned firms in the United States has surged 45 percent, according to the Census Bureau. That’s more than double the growth recorded from 2002 to 2007.

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But female entrepreneurs still disproportionately struggle to draw investors, a phenomenon researchers say is fueled by gender bias. Over the past 15 years, the amount of early-stage investment in companies with a woman at the helm has increased from 5 percent to 15 percent, according to a 2014 study.

Schulte said even this modest growth is enough to inspire others.

“What’s really interesting about this moment in time,” she said, “is there are more women like me, people who never thought about being entrepreneurs, seeing other people do it and feeling encouraged to do it. That really helps you take the leap.”

The desired market

Agrawal’s Thinx and Schulte’s Flex are aimed at a generation rejecting the ancient menstrual stigma — the old taboos that displaced women to special huts during their periods or deemed them unclean during their monthly cycles.

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When Donald Trump lashed out at Fox News anchor Megyn Kelly by saying she had “blood coming out of her wherever,” young women on social media responded with a new Twitter trend: #PeriodsAreNotAnInsult.

“Menstruation is one of those bodily processes women have been obligated to keep hidden,” said Tomi-Ann Roberts, a psychology professor at Colorado College, “and now we’re celebrating it.”

Ann Fishman, author of “Marketing to the Millennial Woman,” said: “You’d have to go quite a distance to hit ‘inappropriate language’ with young women. They want you to treat them as the strong people that they are. ‘You’re a woman. You have periods. Let’s just deal with it.’ ”

On the Internet, young women are lauding the menstrual products they love: “Free bleeding into my @SheTHINX and feelin powerful,” one Twitter user typed last month to her 10,700 followers.

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America’s millennial women, a cohort of about 40 million, embrace the frankness, Fishman said. Savvy marketers are taking advantage of it.

Still, disposable pads and tampons remain the country’s top-selling menstrual products. According to the most recent numbers from the Centers for Disease Control and Prevention, 62 percent of American women said they used pads, while 42 percent said they used tampons. (These shares include respondents who said they used both, but the survey didn’t ask whether they used just one product.)

Industry analysts, though, see potential for a gradual change.

Alternatives “still constitute a small portion of sales,” cautions Svetlana Uduslivaia, head of tissue and hygiene at Euromonitor International, a global market research firm. (Note: Analysts don’t yet track the sales of most lesser-used products on the market, so aggregate data is lacking.) “They are, however, products to watch for long-term impact. As more women, especially younger women, are getting familiar with the products, they are likely to be passing on this information to their daughters.”

Lunapads, a Canadian firm, didn’t see wild success at the jump. It began selling washable pads in 1993 to a small niche of buyers: Women who for medical reasons could not use tampons; and those who did not want to overstuff a landfill. But last year, sales leapt 40 percent, according to company.

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“I knew this was really blowing up when Target called,” said co-founder Madeleine Shaw of her first deal with an American retailer, projecting the deal will help boost the company’s vendor sales by 180 percent. “Our pads go on sale there later this year.”

Diva International, the creator of the DivaCup, a reusable, silicone, goblet-like insert that collects blood, has also experienced a sales surge. The product has been available since 2003, but sales have grown by more than 150 percent since 2014, said spokesperson Daniela Masaro. (An earlier menstrual cup design emerged in the 1930s but didn’t catch on as its disposable rivals have.)

“Customers are looking for products that will improve their health and the environment,” Masaro said. “The category of feminine hygiene is no exception.”

Though there’s no evidence the materials in traditional tampons threaten women’s health, alternatives branded as all-natural seek to capi­tal­ize on swelling consumer interest in organic products. Most tampons sold in the United States are made of primarily cotton and rayon, according to the FDA Food and Drug Administration. Companies aren’t required to list their ingredients.

Lola, a start-up that delivers 100 percent cotton tampons to your doorstep, is seizing on this uncertainty. The company, which launched last year, employs nine full-time workers, all women, and ships boxes to subscribers in 48 states. Lola’s Facebook video questioning standard tampon safety, posted in August, has garnered more than 2.5 million views.

“We exercise, we eat well, we try to take care of our bodies,” co-founder Alex Friedman, 33, says. “And yet here we were, every single month, using a product that we have no idea what was in it.”

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Ranit Mishori, a family doctor in Washington, D.C., occasionally hears these concerns from young women and discounts them. Her only cautions: Avoid products with fragrance or artificial dye.

“We don’t have enough evidence to say tampons and pads you can find at the pharmacy are harmful,” she said. “So I say, use what makes your life easier, and don’t listen to people trying to sell you a specific product.”

At Fashion Week

As her company neared its two-year birthday, Thinx creator Agrawal rented a 13,000-square-foot warehouse last month for New York’s Fashion Week. Like the other designers in town, she planned to flaunt apparel and stoke publicity. Unlike Marc Jacobs or Ralph Lauren or Alexander Wang, her team posted online an open invitation: “This ain’t your typical fashion show, y’all.”

On a muggy September evening, a couple hundred women and a smattering of men waited outside the slate-gray entrance.

An accountant hoped to snag a pair of hip-huggers. A boyfriend anticipated free drinks. Diarra Payne, 23, a Manhattan copywriter, said she came for creative inspiration. She remembered how Thinx had attracted attention last year with New York City subway posters that proclaimed “Underwear for women with periods” alongside pictures of grapefruit segments positioned slices posed to suggest female genitalia. Metropolitan Transportation Authority boss Thomas Prendergast called the ads “offensive.”

Payne calls them honest. “It’s nice to be real with people,” she said, adding that menstruation isn’t obscene to her. “My friends and I say, ‘Yay, I got my period. I’m not pregnant.’”

In one of the company’s online ads, an IV bag drips red fluid into the underwear of an actress as she acts out an afternoon of activities: reading, exercising, eating an orange. “It feels like you’re eating for two,” the narrator says, explaining a side effect of menstruation matter-of-factly. “Except not. Because you’re on your period. So there’s no baby.”

The ad is called “A day in the life of a real menstruating human.”

Inside the Fashion Week warehouse, models stood on white cubes, wearing white robes with panels of fabric missing to expose Thinx’s absorbent underwear. (They’re washable and made to wear alone or as backup on heavy menstrual cycle days.) One model is a Sudanese refu­gee who moved to New Jersey and became a DJ. Another is a young man, sporting briefs designed for people who’ve made the anatomical journey from female to male but still menstruate.

“These are all people who are moving the narrative forward,” Agrawal told the audience. “For women. For everyone.”

After the show, Joyce Gendler, 26, checked out the merchandise table with her boyfriend.

She grabbed a pair of $34 black hip-huggers tucked inside a plastic bag.

“They can hold up to two tampons’ worth of blood,” the saleswoman said.

Gendler, who works at a New York nonprofit group, fancies herself a modern human, unbothered by descriptions of bodily fluid. Still, she later told her companion, “Hearing her say that made me uncomfortable.”

She blamed society and handed over her debit card anyway.

*A previous version of this article featured an incorrect number of teaspoons of blood the Thinx underwear can absorb.