Brigitte, 54, broke the news with pictures on her official Instagram, and asked fans for "positive vibes".
Brigitte already has sons, Julian, 34, Killian, 28, Douglas, 25, and Raoul, 23. All are now in their twenties and thirties.
Congratulations to the happy couple! "Best of luck, love, and happiness to your family", and another gushing, "you look happy and attractive", with a smiley-face emoji. Aside from her social media messages, the actor has not publicly commented on the pregnancy.More news: Nick Jonas and Priyanka Chopra dating?
Nielsen didn't reveal how far along she was, but the Daily Mail speculated from the Instagram posts that she's at at least eight months. "Family throw back", she wrote, alongside a "proud mom' hashtag. Lovely to see your children", said one follower.
Brigitte tied the knot with fifth husband Mattia in Malta in 2006, following a ceremony held in the Dominican Republic the previous year that hadn't been legally binding. Fair play to her, we say.
In 2011, Nielsen, who has appeared on reality TV shows including "The Surreal Life" and "Celebrity Big Brother", told The Guardian that she was hoping to have another child, but felt torn about doing so because of her age.