Bjorn, 26, is Chelsea's first signing of the January transfer window and could be their last under coach Emma Hayes, who has agreed to take charge of the United States Women's national team in the summer. Chelsea have been pushing to sign a new centre-back to provide cover for Jess Carter, who has had to fill in despite being naturally a full-back, and her Lionesses team-mate Millie Bright, who is unable to play every game.