Sources say the boy was sick with pneumonia upon being taken to the emergency room.
"My precious baby boy is so strong", she wrote.
Both Kardashian and West stayed by their toddler's side over the two days, including sharing overnight duties, a source told the gossip site.
Kim Kardashian West ended 2017 on a "scary" note as she watched her young son, Saint, battle pneumonia.
Kim Kardashian has thanked doctors and nurses after her son was hospitalised for three nights following a pneumonia diagnosis.
As action-packed as their holidays were, things aren't about to slow down for the Kardashian Wests, either.
He was then released and is now at home, doing a lot better, insiders claim.
All was back to normal on New Year's Eve, as Kim and Kanye took selfies and posted them to Instagram, showing off her bleached hair while the couple partied at a club and rang in 2018. The pair will become parents of three when their surrogate gives birth to their baby girl, reportedly later this month.
But it really does look like Saint has recovered a lot as his anxious parents managed to spend some time away from the sick boy during New Year's Eve. Had us waiting 25 days to see if Kylie Jenner pregnant and she's not even in the damn Christmas card s***.