You may not know Cecile Reinaud, founder of UK-based maternity wear line Seraphine, but you definitely know her clients.

Seraphine’s designs have been worn by pop stars and actresses including Gwen Stefani, Carrie Underwood and Kate Winslet. But Reinaud’s real claim to fame is dressing the Duchess of Cambridge, a.k.a. Kate Middleton. The young royal wore Seraphine during both pregnancies–including the dress she chose for her first official portrait with Prince George.

The brand’s adoption by Middleton and other celebrity trendsetters has done more than land Seraphine on the cover of some gossip mags. “Celebrities have made being pregnant fashionable,” says Reinaud. “They changed the perception that you are big and you should just hide.” When Middleton is photographed in a Seraphine item, it sells out almost immediately–a phenomenon Reinaud calls “the Kate effect.”

And it’s not just the paparazzi, says Reinaud. Celebrities often boost the brand’s profile by putting their own Seraphine photos on social media–a priceless form of marketing that doesn’t cost the company a dime. Seeing famous women in the clothes also reassures potential customers about the brand’s quality, an important consideration for a company that does 60% of its sales online.

While it expects to remain primarily an e-commerce brand, Seraphine is expanding its brick-and-mortar presence in the U.S. It already has one store in Manhattan, and is in the midst of opening a second. In the next three years, Reinaud says she plans to have 10 American locations. Seraphine is also available in stores like Nordstrom and A Pea in the Pod, which currently account for about 20% of sales.

While still a small player, the 70-person company is growing quickly. Seraphine’s total sales reached $17 million in the last fiscal year, up about 40% over the previous year, says Reinaud. She’s also grown the offerings, rolling out a higher-end “Luxe” line and starting ShoeTherapy, a footwear company focused on chic but comfortable shoes. (ShoeTherapy products will be available in the U.S. in early 2016.)

Reinaud says that when started the company in 2003, the options for fashionable pregnant women were essentially nonexistent. “In Europe there absolutely was nothing,” she says. At the time, Reinaud, who was born in France, was living in London and working at an advertising agency. “I witnessed all around me at the agency these women who were getting pregnant and saying, ‘Oh, no! What am I going to wear? There’s nothing!” she recalls. She spotted a market gap and decided to take the leap.

Of course, the market for maternity wear has expanded dramatically since then. Reinaud says Seraphine continues to distinguish itself by offering a wide range of styles at a mid-market price. The average dress costs around $90. “When you’re pregnant, you have to buy yourself a bit of a new wardrobe, so you’re going to need a couple of pieces,” she says. All the Seraphine designs are also intended to be nursing-friendly, says Reinaud: “I’ve been a mum two times, so I can relate to what you need.”