Actress Scarlett Johansson's pregnancy was a "welcome surprise", says a friend.

The `Avengers Assemble` star announced earlier this week she is having her first child with fiance Romain Dauriac, but a friend says they have known about it for some time, reports contactmusic.com.

"She's about four months along. The baby was a welcome surprise,` said the insider.

The actress had previously talked about her desire to start a family at the end of last year.

"I would like to have my own family, that would be nice," she had said in December 2013.