Heather Rae El Moussa wore a backless dress with a custom 'Baby El Moussa' chain to her gender reveal party

• Tarek and Heather Rae El Moussa had a gender reveal party on Sunday.
• Heather wore a white MadebyILA dress with a custom "Baby El Moussa" chain on the back.

Heather Rae El Moussa had fun with maternity fashion at her gender reveal party.

Heather, 34, and Tarek El Moussa, 40, are expecting their first child together. They announced the news on July 13, revealing that they got pregnant naturally while in the process of IVF.

The El Moussas had a gender reveal party on Sunday planned by Sonia Studios, where they found out they're having a boy. Their son is due in January.

Heather embraced the party as an opportunity to have fun with maternity fashion, wearing an MadebyILA slip dress in a look styled by Caitlin Jaymes.

The white dress had an ankle-length skirt, halter neckline, and floral detailing on the collar.

The dress was backless, with a crystal chain hanging above the skirt. Letters hung from the chain, spelling out "Baby El Moussa."

Heather wore her hair in an updo, letting the fun detailing shine.

"Ortal Mizrahi, the owner of MadebyILA, and I wanted to create a sophisticated and fun dress for Heather to wear for the big gender reveal party," Jaymes said of the look. "As Heather's stylist, I like to always make sure that an outfit can encompass Heather's personal style: sexy, glam, feminine, and classy. When it came to the dress, Ortal and I collaborated on a design that would capture all four characteristics."

"Our sexy factor was the low back, which was balanced out perfectly with the high neck," she went on to say. "The silky ivory fabric gave a sophisticated and classy element and the white rose made the dress feel ultra feminine."

"The star of the show was definitely Ortal's signature crystal chain that said 'Baby El Moussa.' Ortal wanted to make sure this piece was special and personalized for Heather and the chain was the perfect way to add a glam factor to the dress," Jaymes told Insider.

Heather previously told People she was looking forward to wearing fun maternity looks, particularly because she had been hiding the pregnancy.

"I'm so excited to announce it. I've been having to hide it at the gym and at events and going out places," she told People. "I'm used to wearing tight clothing, I'm used to wearing tiny clothing, and having to hide it, it's just been very, very challenging."

"I can't wait to start wearing my tight dresses, my cute maternity stuff, and show it off rather than hiding it," she went on to tell the outlet.