The 54-year-old actress and model recently posted photos of herself sporting a baby bump on her Instagram account.
"Family getting larger", Nielsen wrote on her social media page, as she cradled her growing bump. On Wednesday, May 30, she shared a similar snapshot, writing, "Happy time positive vibes #happyness #positivevibes".
Following Nielsen's pregnancy announcement, a number of fans flooded her recent Instagram posts with messages of love and support, with those hoping to have kids later in life calling her an inspiration.
The publication also said Nielsen had been seen lunching in West Hollywood the last few months and did a good job of concealing the pregnancy.
Nielsen was married to Sylvester Stallone from 1985-87, but she and the "Rocky" actor didn't have children together.
She first came to the public's attention in the early 1980s as a fashion model and appeared in Playboy several times.
The two have been been married since July 2006 when they tied the knot in Malta, Italy after first having an unofficial ceremony in February 2005.
Nielsen was also married to Kasper Winding from 1983 to 1984, Sebastian Copeland from 1990 to 1992 and Raoul Meyer from 1993 to 2005.
54-year-old Nielsen has four other children too. "I'm young at heart and I would be different this time round", she said.