The couple met on the set of the 2008 thriller Jumper and became engaged a year later. They broke up in 2010, but reconciled three months later.
Ms Bilson has previously spoken of her desire to start a family with Mr Christensen, 33, and according to US Weekly, all of their friends and family are thrilled for them.
“They’ve both talked about this for a while and are so excited,” the source said.
In an interview with Cosmpolitan in 2013, Ms Bilson revealed she couldn’t wait to have a family.
“I definitely have family on the brain. Having kids is something I want to see in my future – I hope,” she said.
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