Of all the women who saw the Craigs­list ad asking them to strap on their best bra and go to a loft in downtown San Francisco, 100 showed. After hiking up three flights of stairs, each was told to take a photo of her breasts with an app that could calculate her bust size.

What might sound like a dodgy film audition was actually a highly technical R&D session run by a woman with an MBA from MIT. "Women literally emailed to ask, 'Is this safe?' " recalls Heidi Zak of the experiment three years ago, when she turned the bare-bones office of ThirdLove, the new online bra company she co-founded with her husband, Dave Spector, into a brassiere laboratory.

At the time, Zak was hoping the participants--women ranging from age 17 to 70, and with cup sizes from AA to F--could help refine ThirdLove's virtual bra shopping app. Most women have experienced one or many of the indignities of a poorly fitting bra, including saggy cups, itchy tags, pokey underwires, and a band so tight that it causes ripples in one's back or, more unflatteringly, in one's boobs. Worse than shopping for a bra in a store--the endless hunting for the perfect fit, the handsy saleswomen--was shopping virtually, where you couldn't even test the intimate and finicky under­garment. "I always dreaded bra shopping," says Zak, 37, who was setting out to fix all of this. By using her startup's body-scanning technology app, women could, theoretically, get a bra fitting in the comfort of their own home as accurate as one from a seamstress. Then, ThirdLove could use the fit data of all its customers to design and build the perfect bra.

During the 100-woman focus group, Zak thought her biggest insight would be about the app. Instead, she realized something that would eventually come to define her new business--the bras themselves needed serious rehab. Participants tried on ThirdLove's bra prototype, which Zak had constructed using the industry's standard cup sizes. "It turns out, 37 percent of all women fall between cup sizes," says Zak, who herself had alternated between two cup sizes her entire adult life. "That helped explain why my bras never fit."

For years, the industry's biggest brands had based the molds of their cups on a single pair of breasts. Dorothy Galligan was a 1970s cabaret singer-turned-bra model who had the most in-demand bust for decades. "Dorothy was what the industry deemed a perfect 34B in terms of circumference, shape, and the way she filled out a bra," says Zak, explaining that brands would size up or down from there. In fact, ThirdLove had hired Galligan to size its first bra prototype, but that day Zak concluded "it's not about Dorothy. It's about real women."

That insight has since drawn investors like Laurie Ann Goldman, who arguably knows the undergarment industry better than anyone. "I get pitched so many ideas that it's rare for me to even pick up the phone to follow up," says the former CEO of Spanx, who's based in North Carolina. "But after I spoke to Heidi, I got on a plane."

Zak is now part of a cadre of female entrepreneurs working to upend the panty drawer. Bras, panties (see "A Monthly Makeover," below), condoms, and tampons are being reimagined, remade, and marketed by women who have long used these products and recognize that each is a multibillion-dollar opportunity. "These industries are so antiquated, but nobody's approached them because it's very hard to dynamically change an industry or really innovate if you don't inherently use it and understand it at its core," says Zak, who is also an angel investor in female-oriented condom startup Sustain. (See "Cleaning Up Contraception," below.) Heidi Messer, an entrepreneur who holds all-female poker games that have introduced early-stage female investors to female entrepreneurs, cautions those who write off these types of companies as niche players. "People talk about Dollar Shave Club like, 'Where did they come from?' That business is no different than ThirdLove," says Messer. "They have the same underlying approach to a market that has not been shaken up in a way to match what the consumer wants."

It turns out, though, that trying to sell a bra online is a lot more complex than selling a razor. It has, in fact, long been considered impossible. The same was said about diamonds and mattresses, but Blue Nile and Casper have proved otherwise, and in recent years, half a dozen upstarts have emerged to do likewise with bras, a $13 billion industry long dominated by Victoria's Secret.

Startup True & Co. uses algorithms to match shoppers with bra styles and AdoreMe is a bra subscription service, but ThirdLove's singular bet is on precision. A bra has a lot to be precise about. Whereas a button-down shirt has only four components, explains Zak--fabric, buttons, interfacing, and thread--a bra has approximately 30 components. "Had I known how complicated it was to do what we're currently doing," says Zak, "I never would have started it."

In 2012, Zak was working at Google as a senior marketing manager on its retail sales team when she decided she wanted to start her own company. "I always liked the tangible element of physically selling something and understanding how you build and scale a brand," says Zak, who before Google had spent four years at Aéropostale after stints in investment banking and at McKinsey. As the teen retailer's director of new business and international, she led its overseas expansion and experienced the challenges of trying to change the behavior of a large, traditional company. "Retail is very slow to adopt new technology in terms of how product is developed as well as just how to use technology to create a better experience," she says.

Zak quickly lasered in on bras as a product in desperate need of an overhaul. "My friends and I all had a favorite brand for almost everything--shoes, jeans, beauty products," she realized, "but not bras." She tested the idea with her husband, Spector--the two met while getting their MBAs at MIT--who was then a venture partner at Sequoia Capital. As an investor in the online accessories company Stella & Dot, Spector had seen "the power of women selling to other women, as well as their buying power." The opportunity suddenly became obvious: Women account for over 70 percent of all U.S. spending, and virtually every one of them wears a bra.

Dorothy Galligan was a 1970s cabaret singer-turned-bra model who had the most in-demand bust for decades

Zak, who runs 10Ks ("I stopped doing triathlons once I had my two kids"), didn't waste any time before digging into the market research. "Your average woman has two or three bras she wears 95 percent of the time, and then she has eight others that just hang out but don't fit. She never wears them," says Zak. "We believed we could really own the lingerie drawer by having that one high-quality piece." Spector agreed she was on to something huge, and in 2013 left Sequoia to start the company with his wife.

Before they could build technology sophisticated enough to virtually size a woman for a bra without ever touching her skin, they needed to hire someone who knew the intricacies of the product. "Most people think of a bra as a sexual object. It's actually a highly technical garment," Spector, now the company's president, says he soon learned. Zak, the CEO, scoured LinkedIn and found Ra'el Cohen, a designer who had founded high-end lingerie company Luv & Honey, and was now at young women's retailer Charlotte Russe. Cohen had been approached by several startups over the years, but wasn't swayed until she met the couple. "I knew the industry was broken," says Cohen. "I also knew if they could persuade someone like me to leave a good company to join them, they would be able to recruit amazing people and raise money."

But bras weren't exactly standard pitch fare with the venture crowd. "Most guys agreed to meet because they knew me and my track record," Spector concedes. Guys is the operative word, Zak points out. "Ninety percent of the time, you're pitching mostly to men," she says. In pitches, Cohen would unveil the startup's bra prototypes, but more often than not, the discussions would devolve into Mad Men clichés--male VCs calling in their female assistants and junior-level associates to evaluate the startup's worthiness. After meeting with one top Silicon Valley firm, Spector says a male partner told them he didn't want to invest "because 'we invest only in markets we understand.' "

By late 2013, ThirdLove had managed to raise $5.6 million from investors including Andreessen Horowitz and Novel TMT, and finally had the cash to build the company's body-imaging app. ThirdLove hired a small engineering team, but quickly realized it needed to accelerate the development of a tech­nology that had been tried many times before, to little success. Novel TMT introduced ThirdLove to Indi Custom, a startup that was developing a similar smartphone-based sizing technology to customize and mass produce jeans without fit models. Zak and Spector decided to acquire the company and recast the technology for bras.

ThirdLove's designer, Ra'el Cohen, discovered that a detail as seemingly small as a half-sized cup could change a woman's bra wearing experience.
CREDIT: Anastasiia Sapon

Ultimately, it took two years to develop ThirdLove's patented computer-vision-technology app. After a customer scans herself with a mobile phone's camera, the app measures two critical circumferences of her torso--the overbust, which starts at the nipple, and the underbust, just beneath the breast at the ribcage. Those images are then processed without ever being recorded by ThirdLove--"Privacy is of utmost importance," says Spector--and within seconds, a bra size and style is recommended to a customer.

To design the bra, Cohen used the data from those early Craigslist fittings to create the startup's own non-Dorothy proprietary molds. ThirdLove would become the first bra company to create half-sizes, says Zak, offering 30 percent more cup options. The plan was to roll out ThirdLove's product line in the spring of 2014 with seven distinct styles, and then use the data collected--including sales, returns, and feedback--to fine-tune the bras' designs and future orders.

At least, that was the plan. First, the team would have to figure out how to get the bras made.

To construct a bra that fell outside convention meant Zak and Cohen would need to get on the factory floor. A lot. The two began looking for a manufacturing partner not far from San Francisco. Mexico was as close as they could get. "We had to take two connecting flights to Yuma, Arizona, and then drive for an hour to the border to literally walk through the turnstiles and be picked up by someone from the factory, which was a 10-minute drive from there," Zak recalls. No one but the supervisor spoke English, and it quickly became clear that he wasn't all that interested in breaking the status quo. When they asked him to help them create a hook-and-eye bra clasp that was padded, to prevent it from digging into a woman's back, the manufacturer's response? He didn't know of any supplier who could provide it, and he certainly wasn't going to look for one. Zak and Cohen kept pushing--for stronger yet more supple elastic, for a softer yet still supportive foam--but, says Zak, "the manufacturer kept insisting that the customer didn't care."

ThirdLove eventually launched its first collection that spring with seven bras. "Our strategy was to roll out multiple styles to see which one resonated," says Zak. Within three months, it was clear the team had their breakout winner: the 24/7, which not only sold well, but also had minimal returns. They immediately discontinued producing the other six styles, with the goal of making the 24/7 the single best-quality bra on the market at its $68 price point--so comfortable that a woman could forget she was wearing it.

But Zak realized the only way they could succeed in doing this was to find a manufacturer that was as obsessed with the minutiae as they were. "The details in a bra have been ignored by all the big players because they're so focused on cost," says Zak. "Most of them think, 'How do we shave 5 to 10 more cents off this garment?' "

She and Spector began looking for a new partner in Asia, where most lingerie is made. "We did a lot of trips and late-night wine-and-dine dinners with manufacturers," Spector says. But the best factories were at capacity and hardly interested in taking on a new, unknown brand. Finally, in 2015, they found a partner in China willing to attend to design nuances that may not sound revolutionary but can transform the bra-wearing experience--things like printing a bra's size on the padded clasp, which eliminates the need for a scratchy tag. "We worked with their R&D team to create a lightweight memory foam that molds to the shape of a woman's breast and feels like a second skin," says Cohen, explaining how the collaboration has led to one of the company's several proprietary design elements.

ThirdLove's 24/7 now is produced in six styles (and price points), ranging from lace to strapless. But the designs are treated more like software, continuing to be tweaked and perfected. "Our fit gets better as we grow because of the data we collect and what we learn from customer feedback," Zak says.

In pitches, they would unveil early prototypes of the startup's bras, but more often than not, the discussions would devolve into Mad Men clichés

For example, ThirdLove introduced its strapless bra in 2015, but soon discontinued it because there were still fundamental design flaws. "The biggest complaint about strapless bras is the constant tugging up women have to do," says Cohen, explaining that "most strapless bras have cups that are a C shape, which causes what we call the muffin top." Ultimately, Cohen's design breakthrough was softening the curve of the cup so the breast fills it, rather than spills over it. It took Zak's team a year and a half to develop a 24/7 strapless bra they could stand behind, and after launching in August, they've had a hard time keeping up with demand. "We've sold out certain sizes and are scrambling to up production," says Zak.

All this devotion to detail is starting to pay off. While Zak won't disclose revenue, the company ships approximately 50,000 bras a month, and the average customer makes her second purchase within 45 days. Similar to other online direct-to-consumer companies, like razor startup Harry's, which now sells in Target, ThirdLove is using off­line retail as a tool for brand exposure. Last year, it began selling in Bloomingdale's, and will soon sell at Nordstrom.

Marshal Cohen of consumer-analysis firm NPD says ThirdLove's insight--pioneering cup half-sizes--is revelatory. "You wouldn't want to buy a shoe that didn't have half-sizes. Why would you want to buy a bra without them?" says Cohen. He says the company's singular bet on one superior product is a smart underdog strategy. "Think about how Under Armour took on Nike and Adidas," says Cohen. "People thought they were out of their mind. But they went after their base layer business--not everything--and then they built from there. That is why this model has the opportunity to be successful."

Even more telling is the new breed of investors ThirdLove has been attracting. When the company raised $8 million in February, investors included Lori Greeley, Victoria's Secret's former CEO, and Goldman, the former Spanx CEO.

In late August, ThirdLove was gearing up for its third move in three years to accommodate its growing team of 45. A sea of brown boxes had overtaken the office, except for seven bra-wearing mannequins that stuck out like peacocks. Two rolling racks of bras doubled as a divider between the company's office and showroom, not far from Zak's crammed desk. "We've run out of space," Zak says, reflecting with Cohen on how far the company has come since uncovering that seemingly small insight at the Craigslist focus group. After Cohen created the half-size cup prototypes, ThirdLove had invited the Craigslist women back for a fitting. "Everyone was like, 'Finally! A bra that fits!' " recalls Zak of the emotion of that day. "Some even got a bit teary."

"Women founders are starting companies because they have a certain issue they encounter in their life and they think they can create a better experience," says Zak. Now that she's had a taste of it, she half-jokes she's already spotted the next female-focused industry desperate for some disruption. "I got the Medela Symphony [breast pump] because it works fast and is the most efficient," says Zak, who's still breastfeeding her almost 1-year-old son. "But it's the size of a computer from 1982 and is heavy and loud. We make iPhones that fit in pockets--and nobody can make a better breast pump?"

Cleaning Up Contraception

"Gross, fratty, and ego-driven," says Meika Hollender, describing the marketing plastered onto most condoms. Bro-marketing would be logical for condom com­panies except for the fact that they've long ignored a massive business opportunity. "Forty percent of all condoms are purchased by women, and 21 percent of all sexually active single women use them regularly," says Hollender, co-founder of Sustain Condoms, which makes Fair Trade-certified, toxin-free prophylactics with modern, minimalist package design. In 2012, Hollender was getting her MBA at New York University when her father, Jeffrey, the co-founder of nontoxic cleaning-products pioneer Seventh Generation, told her about his decades-long desire to design a condom sourced from rubber that didn't add to deforestation. Meika, 29, explored all aspects of the condom business--from sustainability to sexual-health issues--and realized no one had ever built a condom company targeting women. "We won't ever do a pink box," she says.

Meika Hollender, co-founder of Sustain Condoms, sources her Burlington, Vermont-based company's latex from a Fair Trade-certified rubber plantation in India.
CREDIT: Cait Oppermann

Purse-friendly condoms

Sustain's condoms employ a special chemical "accelerator" in the production process that stops the release of carcinogens called nitrosamines, which naturally occur in latex. "Nitrosamines have been regulated in pacifiers because they're released in moist conditions," says Hollender. "But there's no regulation on them in condoms."

CREDIT: Courtesy Sustain

A Monthly Makeover

Don't get Miki Agrawal started. "A man invented tampons in 1931," begins the 37-year-old, unspooling the 80-year history of period-related product breakthroughs. "But, really, tampons and pads were the last mainstream innovations." Agrawal is a co-founder and the CEO of Thinx, the startup hoping to claim the next great period innovation. Thinx is the creator of "period panties," undies that are setting out to replace a $15 billion industry. Whereas a tampon can leak and maxi pads often feel like a diaper, Thinx's patented Thinx QuadTech microfiber panties--ranging in styles from boy shorts to thongs--can absorb up to two tampons' worth of blood in their antibacterial crotch panel. While each pair of $24 to $38 undies needs to be rinsed before washing, women seem game for the sacrifice. "Women who wear them feel like, 'I'm still a sexy girl wearing hot underwear,'" she says.

"Twenty billion plastic tampon applicators and pads end up in a landfill every year. It's just not sustainable." - Miki Agrawal, co-founder of Thinx and the creator of "period panties," an attempt to unseat the billion tampon industry.
CREDIT: Stephanie Noritz

Food porn

In 2015, Agrawal went head to head with New York's Metropolitan Transit Authority when Thinx unveiled its ad campaign, which used photos of split grapefruits and raw egg. "The MTA found it offensive--but not the breast-augmentation ads," says Agrawal, who's career includes investment banking and semipro soccer. After a monthlong battle, Thinx won.

CREDIT: Courtesy Thinx