When Marissa Vosper started the intimates brand Negative Underwear in 2014, she and her co-founder Lauren Schwab decided not to retouch women’s bodies in the marketing materials.

Ms. Vosper called the move “a no-brainer” — befitting a business that sells mostly sheer, wireless, ultra-minimal bras. There would be no #photoshopfails of the kind that go viral on social media: missing belly buttons, extra arms, thigh gaps that warp a background scene. Skin would have texture. Bodies would have stretch marks.

For the most part, the approach seemed to work. The initial run of Negative Underwear sold out in two weeks. But there have been unintended consequences to Ms. Vosper’s attachment to realism in marketing. A recent ad featuring a close-cropped photo of a woman’s breasts in a Negative bra showed delicate stretch marks whiskering across her chest. Some consumers balked — not because the ad showed stretch marks, but because the stretch marks themselves appeared too pretty and uniform. Ms. Vosper recalled that people were posting statements on social media like: “There’s no way her stretch marks are that perfect.”

For more than a century, advertisements for lingerie have emphasized an idealized version of the female form. In the 1910s and ’20s, ads for shapewear featured hourglass shapes. In the 1940s the underwire bra was embraced widely.

In the 1970s, Rudi Gernreich invented the thong and scandalized the masses. In the ’80s and ’90s, Calvin Klein learned to sell sexiness as much as cotton panties. During the early aughts, attention on Victoria’s Secret reached a fever pitch. The brand traded on near-celestial expectations for perfection. In ads, its “angels” seemed to have not so much as an errant mole, and cellulite was for earthbound civilians.

But in the past decade, and as Victoria’s Secret has stumbled after a series of scandals, newer underwear brands have focused on so-called imperfections — stretch marks, in particular. In the lingerie business, the once-verboten “flaw” is approaching industry standard, as ubiquitous as cleavage.

Several intimates lines including Negative and Cuup have cast models with stretch marks. Some big-box retailers followed suit. Stretch marks (often seen on otherwise thin models) appear in product shots, forASOS, Boohoo, Missguided and Target, among others. Some were earlier adopters: The month before Negative entered the market, Aerie pledged to stop airbrushing lingerie ads.

When Everlane started selling underwear, in 2018, its tagline touted a lack of frills, bows and deception. The lingerie start-up Parade built its reputation in part on raw product images. In August it was acquired by Ariela & Associates — licenser of the underwear behemoth Fruit of the Loom.

Conventional wisdom has evolved fast, but change has been slower behind the scenes. When Cayla O’Connell founded her underwear brand in 2017 (once known as Knickey, recently restarted as Subset), she saw a common denominator in her competition: The most mainstream underwear brands had men at the helm. She wanted to offer a different perspective.

“As a woman, I want to participate in consuming things that speak to me,” Ms. O’Connell said. “And my reality is, I have stretch marks, and I am not a size zero.” From the beginning, she was resolute: “We’re embracing cellulite, we’re embracing eczema, we’re embracing body marks and tattoos, and body modification, and differently abled bodies.”

Ms. O’Connell said she viewed Subset’s commitment to showcasing a diverse cast of models (and stocking sizes up to 4XL) as distinct from the tactics of larger companies. Those brands, she said, are “strategically dipping their toe into the zeitgeist,” with the occasional ad campaign featuring models with stretch marks and other imperfections.

Ms. O’Connell finds that strategy disingenuous. “It’s a direct result of backlash that some of these companies were getting around the objectification of women and sexualization and infantilization of women in media and marketing,” she said.

The Negative Underwear stretch mark sleuthers suggest that even consumers who choose to shop with indie brands are still holding onto some of that latent skepticism. Is it progress if a perceived blemish becomes a trend?

Negative said it does use postproduction techniques on its product shots, including color correction and wrinkle reduction in fabrics, but Ms. Vosper said the brand remains true to its vision. “We don’t alter women’s bodies,” she said. “We’re not going to slim down her waist. We’re not going to remove her cellulite.” Note to the doubters, she said: The stretch marks are real.

Like Ms. O’Connell, Ms. Vosper has watched stretch marks become something of a gateway flaw — the imperfection of choice for more established companies that want to at least nod to inclusivity. Even Victoria’s Secret has experimented with it, albeit waveringly: In 2016, ahead of its annual fashion show, the company released images of the model Jasmine Tookes wearing a $3 million bra, stretch marks included. But when Ms. Tookes walked in the live show, she wore a beaded one-piece that covered her upper thighs — and her stretch marks — on the telecast.

Faren Karimkhan, a professor of advertising at the Newhouse School at Syracuse University, thinks that legacy brands see stretch marks as a “less risky” way to participate in conversations around body positivity. She said that companies like Victoria’s Secret are “more conservative in buying into this new trend.”

“I think stretch marks may be perceived, for them, as a safe place to start,” Dr. Karimkhan said. “After all, even thin and model-beautiful women have them.”

She added: “We all know that brands, specifically in the apparel and cosmetics industry, have presented it as though imperfections — cellulite, acne, stretch marks — just don’t exist.”

When it comes to lingerie, Dr. Karimkhan cited research that indicates women expect wider representation than they do from apparel. “Intimates are an intimate thing,” Ms. O’Connell said.

The success of brands seen as more body positive is evident, Dr. Karimkhan said. “But more traditional brands that have their roots in that ideal image are struggling to find their identity as these new trends emerge.”

Stretch marks have become a low-stakes point of entry, Dr. Karimkhan said. And not only low-stakes, but relatively cheap compared with the process of creating and photographing samples in multiple sizes to show diverse bodies.

The ledger tells the story: Stretch marks have been championed in part because it doesn’t cost an extra cent to have a model with them on set. Brands like Negative and Subset, which do shoot multiple women wearing different sizes of product, pay for that investment.

“We sample everything in extra-large in addition to small,” Ms. O’Connell said. She declined to estimate the additional cost, but said that the brand aims to have each product shown on three women. That means more hours on set, more models, more samples.

Ms. O’Connell compared stretch marks to freckles. Those too were once erased in conventional advertising, but now Sephora and other retailers sell a pigmented pen to help the freckle-free approximate the look. Thus far stretch marks have not been sold à la carte, but Ms. O’Connell sees it as possible.

Would that be a sign of enlightenment? She’s not so sure. At least not when other apparent imperfections are still wiped off brand marketing. For now, she said she still has to reiterate to her photo editors after shoots: “No retouching.”

“We certainly get feedback from customers and followers on social media in both ways,” said Ms. Vosper, the co-founder of Negative. “Some people think it’s never enough and some people think it’s too much, and everyone has a comment when it comes to women’s bodies.”