Scarlett Johansson and Romain Dauriac

Scarlett Johansson and Romain Dauriac

Scarlett Johansson's pregnancy was a "welcome surprise".

The 'Avengers Assemble' star announced she is having with her first child with fiancè Romain Dauriac earlier this week but a friend says they have known about it for some time.

The source told UsMagazine.com: "She's about four months along. The baby was a welcome surprise."

The insider says Scarlett has been looking forward to telling people of her happiness, adding: "She's just excited to share the news."

The actress had previously talked about her desire to start a family at the end of last year, saying in December: "I would like to have my own family, that would be nice.

"They say it's never the right time and I am sure that's true, but I think you have to plan it like anything else.

"At some point it is something I look forward to."

Scarlett, 29, and Romaine, 31, got engaged in August last year after he proposed with a vintage Art Deco ring.

The 'Captain America: The Winter Soldier' star has previously said she and Romain don't want to "rush through" tying the knot, as "being engaged is an exciting time to enjoy and to really savour."

The marriage will be Scarlett's second after she was previously wed to actor Ryan Reynolds between 2008 and 2011.