The concept of “blood” and “bleeding” is generally avoided in mass marketing for period products. It was only recently, and with some fanfare, that commercials showed red liquid being absorbed, instead of blue.

But when it comes to period underwear — an increasingly popular type of underwear made with extra-absorbent fabric — it’s difficult to avoid. At least when talking to the founders of the Period Company, a brand that was introduced in October, touting period underwear that was more affordable and sustainable than other menstrual products. For them, bleeding is a kind of profound act.

“Something emotionally begins to happen when you bleed into your underwear and you don’t have tampons, you don’t have pads, you don’t have waste — when you’re just allowed to really kind of be in your period,” said Sasha Markova, who, with Karla Welch, founded the company.

“Flowing is a very different experience, and we feel kind of evangelical about it.”

Ms. Markova, a longtime creative director, is not exaggerating about evangelizing; she refers to switching to their product as “conversion.” As in “We really converted ourselves to the idea of this underwear.” Or “The amazing thing you can begin to do with Gen Z is say: ‘OK, now we’ve got you. Hey, convert your older sisters and your mothers.’”

There’s a spiritual element to this approach, landing somewhere between typically Californian and harmlessly cultish. But conversion really is essential to running the business. The Period Company and every other brand making alternative products (such as the menstrual cup) needs customers who are open-minded enough to break from the products they’ve always used — the products their mothers handed them long ago, “sighingly, with a lot of burden,” Ms. Markova said.

It isn’t an easy adjustment, particularly when generations of women have been raised to dread leaks. (For a time, fear of humiliation was a hallmark of period product commercials, along with the blue mystery liquid.) And there is growing competition for those willing to convert.

Which is why it helps that the company was co-founded by Ms. Welch, a high-profile stylist whose clients include Tracee Ellis Ross, Olivia Wilde and Sarah Paulson. (On Instagram Chelsea Handler and Busy Philipps were among the celebrities giving unpaid endorsements to the brand, wearing matching gifted sweatshirts that read: “Dear Mother Nature: Thank You!”) Ms. Welch has also designed a line of tees in collaboration with Hanes, initially inspired by her client Justin Bieber, as well as jeans with Levi’s.

Four years ago, when her child’s first period arrived, Ms. Welch found herself in a “hot mess,” struggling to guide her now teen, who doesn’t identify as female, through the traditional options.

“Which made me go back to when I got my period, and my mom didn’t even talk to me about it,” she said.

Still, another 20 percent of respondents said they had never even heard of reusable products until the survey, Ms. Enkema said.

When Ms. Welch turned to period underwear for her child, it was a solution, but it wasn’t perfect. Most pairs ranged from about $25 to $40, and she didn’t want to pay $40 for juniors underwear.

The market’s two dominant brands are Thinx and Knix, both founded in 2013. At one point, Thinx was considered one of the fastest growing companies in the United States. It made headlines for its subway ads and its founder Miki Agrawal, the self-titled “SHE-EO” ousted in 2017 following sexual harassment allegations (which she denied). Another competitor, TomboyX, specializes in gender-neutral underwear, while Ruby Love (formerly PantyProp) was founded to help address urinary incontinence.

The founders of the Period Company said they’re fans of these brands, but, as Ms. Welch has repeated, she and Ms. Markova are more interested in being like Jockey, offering basic no-frills underwear, than like La Perla. Their prices fall between $12 and $14. (Comparatively, a pack of disposable tampon or pads typically costs under $10.)

Their underwear fits tightly but with some stretch, not unlike shapewear, if shapewear had a pad sewn into the crotch between two thick layers of cotton; converting to the underwear seems easiest for those who already rely on pads. There are a few different cuts, including high-rise and bikini. They’re all black, except for two gray junior-size styles. After a day of wear, the product is rinsed in the sink and wrung out, then laundered or hand-washed. Sizes go up to 3X, although the company expects that by the holidays, they will go up to 6X.

“The only way you can really have change is if you’re available to everybody, and you’re affordable and you’re willing to go to a really mass market,” Ms. Welch said. “We don’t want to be posh. We want to be accessible.”