Hollywood actress Scarlett Johansson's pregnancy with fiance Romain Dauriac has reportedly come to her as a welcome surprise.

The 'Her' star is said to be known about her first pregnancy for quite some time now, reported Us magazine.

"She's about four months along. The baby was a welcome surprise!," a source said.

The 29-year-old actress and her French journalist partner Dauriac, 32, began dating in November 2012, and confirmed their engagement last September. On March 3, reports emerged that the couple are expecting their first child.

"She's just excited to share the news!" the source added.

Johansson will continue to shoot in 'Avengers: Age of Ultron', reprising her role as Black Widow.