"Orion is getting a brother- I better get my a** in shape," he wrote next to the snap of a smiling Tara.
His "Sex and the City" co-star Sarah Jessica Parker congratulated the couple over the news, commenting "Oh. My. G-d! I'm so happy for you both!!!"
Chris famously played John James 'Mr. Big' Preston on the hit US show, the love interest of Sarah's character Carrie Bradshaw. He was also congratulated by his "Gone" co-star Leven Rambin.
The veteran actor and Tara, 37, began dating in 2002 and wed in 2012.