Ida Tin wanted to build an app to help women track their periods. For months, she went door to door, pitching the concept to early-stage investors in Berlin, London, New York, and Silicon Valley. Often, Tin, a soft-spoken thirty-seven-year-old from Copenhagen with cerulean-framed glasses, was the only woman in the room. Many potential investors balked at the notion of software with inputs for levels of menstrual bleeding, breast tenderness, and sex drive, or the capacity to digitally share windows of ovulation with a partner. Time and again, Tin told me, the men sitting across from her in pitch meetings said, “I only invest in products I can use myself.” The idea embarrassed even some who saw its business potential; one venture capitalist who eventually made a small investment insisted that his involvement be kept private. But Tin persisted, cobbling together fifty thousand euros. She launched the app, called Clue, in 2013, basing it in Berlin. It quietly amassed millions of active users, primarily in the United States. “This was clearly not a niche product,” Tin said. This past November, Clue announced that it had closed a twenty-million-euro round of funding.

Tin’s experience highlights a blind spot in the global investment community, which has been eager to throw cash at the next food-delivery or ride-sharing app but tends to shun products by and for women—or else hangs back and lets them twist in the wind for too long. When it comes to menstruation-related products, and technologies catering to women’s health more broadly, Tin said, “there’s still a social taboo.” This squeamishness may come as little surprise, given the demographics of the venture-capital world. A recent study of the top hundred V.C. firms around the world found that just seven per cent of active investing partners are women, and that companies founded by women have received just a tenth of global venture-round funding. Yet the demand is inarguably there. The international market for feminine-hygiene products—physical items like tampons and sanitary pads—is projected to hit forty billion dollars by 2020, according to the market-research firm Global Industry Analysts. For decades, the space has been dominated by consumer-goods giants like Procter & Gamble and Kimberly-Clark, which have had little incentive to develop new technologies. And the barrier to entry for newcomers, no matter how scrappy, is enormous. It took more than a year for Ridhi Tariyal, a Harvard engineer, and her business partner Stephen Gire to find investors willing to stomach their invention—a “smart” tampon dubbed NextGen Jane, capable of testing menstrual blood for warning signs of cancer and reproductive diseases.

Some startups survive only by circumventing the traditional investment pipeline. In 2013, Miki Agrawal pitched about twenty early-stage investors in New York on her prototype for period-safe moisture-wicking underwear. All the investors were male. “They found the idea absolutely, horrifyingly gross,” Agrawal told me. “Periods are the oldest shame.” She and her twin sister, Radha, turned to crowdfunding as a last resort. Their Kickstarter campaign for Thinx panties quickly raised sixty-five thousand dollars, far eclipsing their goal. Then a spat with New York City’s Metropolitan Transportation Authority over a suggestive ad campaign went viral, and the company was overwhelmed with online orders. Today Thinx is profitable, Agrawal said, adding that she and her sister are now “being accosted” by investors. Indeed, in recent months, the global venture-capital hive-mind has signalled that its attitude toward menstruation may be changing. A company called Flex, for instance, which makes a disposable disc designed to allow women to have mess-free sex on their period, raised north of four million dollars in the year after it was founded. That should come as good news to entrepreneurs like Tin, but she remains skeptical. “The bar keeps going up,” she said.

In retrospect, Tin told me, she’s glad to have largely struck out with German investors, who she said tend to err on the prudish side. She ultimately raised the majority of Clue’s venture capital from, among others, Union Square Ventures, which is based in New York, and Nokia Growth Partners, which has offices in five countries. Tin fought to keep the company in Berlin, where she lives with her partner and co-founder, Hans Raffauf, and their two young children, turning down funding from Silicon Valley firms that insisted Clue relocate there. After a peripatetic early adulthood leading motorcycle tours through Cuba and Mongolia, Tin was eager to put down roots in Berlin, which she described as less expensive and frenetic than New York or the Bay Area. “I don’t want to accept that you can only build big companies out of Silicon Valley,” she said.

While female tech C.E.O.s face a familiar set of obstacles around the world, the battle is especially tough in Berlin. The German capital increasingly bills itself as a growing international tech hub; it has lately spawned high-growth firms like SoundCloud, a music-sharing platform, and Zalando, an e-commerce site. The city’s cheap rents and dynamic night life are potent draws. Yet, despite its reputation as a bastion of progressivism, Berlin struggles to nurture women founders. Just nine per cent of existing Berlin tech startups were founded by women, compared with an average of seventeen per cent across Europe. In Silicon Valley, that proportion is twenty-four per cent. The female founders I spoke with in Berlin pointed to national family-leave policies and local child-care customs that seem to promote stay-at-home mothering over a woman’s full-time career. (Chancellor Angela Merkel, who holds the country’s highest political office, has no children.) They also told me that efforts in Silicon Valley to foster female leadership in the startup world haven’t yet gained traction in Berlin or elsewhere in Europe, where the handful of programs geared toward women tend to focus on teaching entry-level programming skills. The landscape is still “such a man’s world,” Tin told me. “Just you being there, as a woman, changes the whole thing.”