The "Mamma Mia!" star, who is up for a Best Supporting Actress Oscar for her portrayal of silver screen actress Marion Davies in David Fincher's movie, was expecting her son, who is now seven months old, as she filmed the project, and admits she was so ill on set.
"The weekend before one of the big scenes, where actually I don't have any lines..., I thought that it was going to be a little bit easier (even) knowing that I was going to have to be in the makeup chair at 3.30 (am) on one of the days," Amanda told Julianne Moore during a "Vanity Fair Cocktail Hour, Live!" chat.
"I was in bed the whole weekend thinking, 'I don't think I can do this. I can't be sick and working with David Fincher and play Marion Davies at the same time.' "
Amanda, who also shares daughter Nina, four, with husband Thomas Sadoski, revealed costume designer Trish Summerville was the only person she confided in about her baby news.
"(I said) 'I'm four and a half weeks pregnant, I'm not supposed to say anything but I'm bloated and I can only eat cream cheese!'" she recalled.
Amanda Seyfried starred opposite Gary Oldman in the black-and-white movie. She is nominated for Best Supporting Actress at the Academy Awards while the male lead and the helmer are up for Best Actor and Best Director respectively. The film is additionally vying for the most coveted title of Best Picture.