While there's no news on the baby's gender, from the captions it looks like their new little bundle has a due date of December 19 - meaning the bub will share the same birthday month as its big sister, Mila, who was born on December 5 last year.
Mila is the second child for ex-rugby player Kris, who also shares a son, 8-year-old Ethan, with Dannii Minogue.
At the time of Mila's birth, the proud new dad gushed: "Last night at 10.15pm the world became a better place, the birth of our baby girl Mila Elle has been the most magical adventure ever."
Clearly smitten with his daughter, Kris's Instagram profile is filled with photos of the proud dad and his little girl.
While Kris and Sarah - who have been together since 2017 - are yet to confirm rumours of an engagement, they sparked speculation last month after Sarah unveiled a gigantic diamond ring on her Instagram.
Congratulations to Kris and family!