Mom-to-be Khloe Kardashian's trip to Japan this week with sisters Kim and Kourtney has caused quite a commotion with Keeping Up With the Kardashians fans around the world. But Breaking Celebrity News has learned the 33-year-old's baby daddy, Tristan Thompson, isn't so thrilled about the far-off getaway!

"Khloe was told to slow down on numerous occasions, but she insisted that she go to Tokyo with her sisters for a promotional gig," a source said, adding that the Kardashian gals are also filming for the upcoming season of KUWTK while in Tokyo.

"Tristan did not want her going at all. But she didn't listen, and told him there was just no way she could pass it up."

According to the insider, "Tristan is just worried sick right now. He feels like she could have put their baby's life in danger by flying so far away, especially since it is not too uncommon for women to go into labor early."

As Breaking Celebrity News previously reported, the 26-year-old Cleveland Cavaliers star wants Khloe to quit KUWTK to be a stay-at-home mom after the birth of their son, and live a more balanced life in Ohio.