The happy newscomes weeks after Amanda revealed her surprise engagement to her Last Word co-star Thomas Sadoski - just six months after they started dating.
A source told Us Weekly magazine at the time: "Tommy and Amanda recently got engaged and shared the happy news with family and close friends."
In March, it was revealed the couple had started dating after reuniting on the set of US drama movie The Last Word, in which Amanda portrays young writer Anne Sherman.
The blonde star and the 40-year-old actor first met in 2015 while co-starring in Broadway's The Way We Get By.
Their marriage will be the first time Amanda - who has previously dated Justin Long and Dominic Cooper - has walked down the aisle, but it will be Thomas' second marriage.
He was previously wed to wife Kimberly Hope for eight years, but they split in the summer of 2015.
The baby will be a first child for both.
Amanda and Thomas were first romantically linked to each other in March before their romance was confirmed in May.
Amanda Seyfried and Thomas Sadoski are expecting their first baby shortly after getting engaged. Photo / Getty Images
As well as Justin - who she was in a relationship with for two years - Amanda appears to have a penchant for actors as she has dated the likes of Josh Hartnett, Ryan Phillippe, Alexander Skarsgard and Desmond Harrington.
The actress has previously admitted she won't wear white on her wedding day because she has been involved in so many nuptials through her acting roles.
She said: "I got married so many times in my life - on-screen. I don't want a white dress! I've worn so many of them.
"The fact is, that kind of stuff is less about the ceremony and more about the commitment. So it's like children. That's where it's at, right? For me. But it's different for everybody."
