A model wears a look from the C'est Nous by Shoshanna label.

In the 20 years since Shoshanna Lonstein Gruss started her dress company, she married, had three children, added swimwear, eveningwear, a girls’ collection and a boys’ one, launched e-commerce, divorced and now will launch an exclusive label with Gwynnie Bee Sunday.

From the start, the designer has offered lingerie-inspired sizing for shoppers, so this latest pairing is a logical move. C’est Nous by Shoshanna is the designer’s first fully inclusive line, ranging from XS-5X. A UCLA grad with a double major in history and art history, her original plan was to take a job in finance. Like legions of other college grads, she spent her last free summer traveling in Europe with a friend. Gruss started sketching on the trip and never stopped. “Honestly, in the beginning what I imagined was driving around the country with my samples and showing them to the stores that I thought I should be in. Road tripping — that is really what I wanted to do,” she said with a laugh. “Then I was like, ‘Damn it. I have to work every day. I don’t like this, but I got used to it.”

Her parents loaned her $30,000 to produce the first collection, on the condition it was rent money or that. So Gruss lived at home and eventually paid her parents back. “I was like, ’This is pretty sweet. I work all day, I go out all night, someone does my laundry — I’m staying here.”

Five years later she moved out, when she married Joshua Gruss. Her fashion career goes back to her undergrad days, working at Tracey Ross’ Los Angeles store, when Robertson Blvd housed about 20 furniture stores, The Ivy and Agnes B. “She kind of curated her closet, which was new at that time. I had only seen department stores. There was no Scoop, no Intermix,” Gruss said. “Still, nothing fit me — I loved the store, I loved the girls and I worked there for the discount pretty much.”

It wasn’t until seeing Hard Candy founder Dineh Mohajer and other entrepreneurs walk into Tracey Ross with samples to pitch the owner that Gruss realized, ‘Oh, you can do this. That’s really where the seed was planted,” she said. “I wasn’t thinking, ‘Oh, I want to do this.’ But I thought, ‘Why isn’t anybody making clothes for me? I’m young, I have money to spend, I like my body but no one seems to address it. It was the time of slip dresses and grunge. You had to look like you were 11 forever. It was so defeating so it was the right time, right place, right mindset. Certainly, it was a different world. There was no Internet, no H&M, no Target. Now, there is just so much more competition but I think I’ve gotten better.”

With her 6-year-old twins now in kindergarten, Gruss, who also has a 13-year-old daughter, also has more time for work. As her e-mail inbox and Instagram feed can attest, many shoppers have been requesting larger sizes for a while. The Shoshanna label has offered them on a limited basis due to distribution, according to Gruss, who noted that some accounts will only buy up to a size 8. “I know how frustrating it is. I used to say that stores were like museums. I appreciated it, I could look at it, but it really wasn’t for me and I couldn’t have it.”

Separates may be added down the road, but the first collection has 12 styles of dresses retailing between $118 and $160. The designers and Gwynnie Bee executives declined to comment on projected volume. Eager for feedback from Gwynnie Bee customers, Gruss said she will use that information for the next collection. “Obviously, my [own] brand was started to be more inclusive than the 0 through 12 range. This seemed like a natural progression,” she said.

Her own label is offered in 400 stores including Neiman Marcus, Saks Fifth Avenue, Shopbop and Bloomingale’s. Next month, an exclusive collection will debut at Saks. “We’re sort of celebrating our 20th all year, because why not?”

Convinced that going with your gut and challenging the traditional way of doing things are essential, Gruss said, “I read newspapers, magazines. I’m fascinated by our global history currently and what’s going on. If you pay attention to everything and everyone, you can keep your finger on the pulse and know what’s going on. Then you can pull back when you need to or go forward in a certain way.”