January 24, 2017 09:15 AM

Orange Is the New Baby Bump!

Laura Prepon stepped out at the Sundance Film Festival in Park City, Utah on Sunday — showing off her growing belly in a snug black sweater, slim jeans, a fur-trimmed green winter coat and a grey hat.

The 36-year-old Orange Is the New Black star is expecting her first child with fiancé Ben Foster, a source confirmed to PEOPLE on Monday.

The low-key couple were engaged in October. Though they’ve kept their relationship quiet, the expecting parents have known each other for 18 years.

RELATED VIDEO: Laura Prepon and Ben Foster Expecting First Child

Foster was by his fiancée’s side on Sunday, as the two attended the Creative Coalition’s annual Spotlight Awards at the Kia Supper Suite in the evening.

“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 told PEOPLE. “Neither of them got up much to mingle. They definitely enjoyed dinner. I noticed she finished her plate.”

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.

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