Theia designer Don O'Neill has been having night sweats since December. His first-ever runway show is Wednesday, and things keep going wrong.

A week before the show, an important gown arrived with its elaborate embroidery completed perfectly—all on the wrong fabric. An Italian wool-jersey fabric shrunk dramatically each time it was ironed. Models have been chosen—and dropped out. "There are so many variables that are out of your control," says Mr. O'Neill. "All of those jigsaw pieces have to come together."

More than 300 designers are showing their collections on the runways this week in New York before the shows move on to Europe. That's more than will show in London, Milan and Paris put together. Chalk it up to American entrepreneurial spirit and the lack of a controlling body overseeing the shows. In Europe, major fashion weeks are governed by national councils that control who can show and when.

Fashion consultant Robert Burke estimates that out of the 308 labels showing in New York, only 40 have annual revenue of more than $10 million. That means the other 85% of labels are effectively mom-and-pops with big dreams.

Among the handful of first-timers hitting the runway this week are Theia, Billy Reid, Suno and Levi's.

Last year, Billy Reid, Suno and Theia held informal presentations for fashion editors and buyers instead. Jenne Lombardo, fashion director at Milk Made, a production house that backs emerging designers each season, says editors and buyers prefer presentations for their ease and speed. It takes an hour to see a 15-minute runway show, allowing time for the mechanics of seating and other preparations. But a 15-minute presentation takes just about that.

Even so, the allure of the runway is hard for designers to resist. Len Peltier, Levi's creative director concedes that he worries about seeming "presumptuous." "We're excited to launch something on the world stage," he says, but "we know we're not a high-fashion brand."

Runway shows create more opportunities for video, and photographers can snap pictures from risers at the end of the runway—making it easier for labels to use the show later in marketing.

But for all their promise, runway shows are full of heartache and fear. Mr. O'Neill, the designer of three-year-old Theia, has seen his designs on celebrities such as Angela Bassett and Taylor Swift. For his own show, he dreamed of a Plexiglas runway glimmering beneath the evening gowns for which the label is known. Then he had to nix the Plexiglas when the cost estimate arrived: $7,000. His runway will be covered in white carpet.

The designer, whose label is privately held, says he doesn't have a firm budget for the show, but he knows he shouldn't spend too much. He tries to cut corners where he can. The show is set to take place Wednesday afternoon at a venue 40 blocks from the Lincoln Center tents.

One job of a runway-collection designer is to wow the jaded fashion magazine editors in the audience. This crowd is looking for signals that a designer has buzz and artistic talent—and for ideas for their own pages. Often, pleasing them requires designs that are very different from what a designer might present to shoppers.

Yet Mr. O'Neill worries that in his haste to create dramatic fashions in fine fabrics, he may have created several too-costly looks, including a lambskin poncho with a Mongolian lamb collar. "Either I get fired after the show because I spent too much," he says, "or it'll be a huge success and I can keep my job."

Another faction to please with his collection: his sales team. "Why is it so dark?" asked one saleswoman, referring to a number of looks in black. "Because that's what I'm feeling," he said.

A week before his show, Mr. O'Neill said he had completed about half of the 30 looks he was aiming for.

Getting models has been a particular problem. Mr. O'Neill settles on a model, and then her agent warns her to hold out for a bigger label. "If you aren't Marc Jacobs, it's hard to get models. And I'm not Marc Jacobs," Mr. O'Neill says.

Despite the stress, Mr. O'Neill says he is counting on results. "There are some publications that won't even come look [at his presentations]. So I'm hoping that this will get their attention."

Being the lead designer has helped him look back with sympathy at his previous bosses at other brands, such as Carmen Marc Valvo. "At Carmen, I'm now understanding why he was how he was before a show," says Mr. O'Neill. " Very, very stressed out."