Laura Prepon and Ben Foster Expecting First Child

Laura Prepon is pregnant!

The Orange Is the New Black star, 36, is expecting her first child with fiancé Ben Foster, a source confirms to PEOPLE.

The news comes a day after the couple attended the Creative Coalition’s annual Spotlight Awards at the Kia Supper Suite during the Sundance Film Festival on Sunday night.

“Ben seemed really protective and kept very close to Laura the entire night. He was holding on to her arm as they walked in and kept by her side during the dinner,” an onlooker tells PEOPLE. “Neither of them got up much to mingle. They definitely enjoyed dinner. I noticed she finished her plate.”

Though sources confirmed to PEOPLE that the couple were engaged in October, the low-key duo, who have known each other for 18 years, had kept their relationship quiet.

In fact, even Prepon’s friend and former That ’70s Show costar Ashton Kutcher was out of the loop.

“I get it. But private lives include friends!” Kutcher joked to Prepon on an October episode of Live with Kelly, three days after her engagement news went public. “Like, I shouldn’t find out when I get off the plane!”

“This little baby just got engaged. Over/under on me crying at the wedding … ” another of the actress’s former That ’70s Show costars, Danny Masterson — who is a longtime friend of Foster, 36 — captioned a throwback Instagram snap of the pals from their time on the show, which ran from 1998 to 2006.

Prepon has starred as Alex Vause on Netflix’s critically acclaimed series Orange Is the New Black since its inception in 2013, and appeared in the Emily Blunt-led thriller The Girl on the Train last year.

Foster has acted consistently over the past 20 years — most notably with roles in films like 3:10 to Yuma, The Messenger and Hell or High Water, as well as for his run as Russell Corwin on HBO’s early-2000s hit Six Feet Under.

A rep for Prepon could not be reached.

With additional reporting by PATRICK GOMEZ