While developing her apparel company Spanx, Sara Blakely found she was often being underestimated. It's an issue that affects many women entrepreneurs, she says--for both good and bad.

"In the beginning I wasn't taken seriously," Blakely said Monday at the Inc. Women's Summit in New York City. "It was very difficult for me to even get my product off the ground."

Although it was frustrating that no one would give her a chance at the time, she added that being underestimated can also give women a competitive edge.

Blakely also spoke about building a billion-dollar brand at the conference and touched on using her business to advocate for women. "Professionally, I'm most proud of forcing an industry that stopped caring about how we felt in things and forced them to start making things better for us," she said.

Like many women, Blakely said, she grew up hearing the phrase "beauty is pain." As a response to that concept, which she called "B.S.," in 1998 she cut the feet off a pair of pantyhose and began her shapewear company. Spanx is now worth more than $1 billion and has expanded into leggings, hosiery, and other products.

Blakley announced Spanx's newest offering, called arm tights, at the Women's Summit. She described the product as a crop top with long sleeves that is made from the same material as the company's famous tights. Blakely, who modeled a sparkling black version, said it will allow women to wear sleeveless tops in the cooler weather. (You can see the entire announcement in the video below.)

"It completely transforms your wardrobe," she said.