Kardashian shared a photo of herself wearing a fur coat and a clingy black dress that emphasized her baby bump.
The picture had received over 2.6 million likes as of Wednesday morning.
"Officially 6 months," the mom-to-be captioned the snapshot.
Kardashian also posted a photo of herself sharing a kiss with Thompson, who is already parent to 1-year-old son Prince with Jordan Craig.
Kardashian has been linked to Thompson, a 26-year-old Cleveland Cavaliers player, since October 2016.
"Mom and Dad," the star captioned the picture.
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Kardashian announced her pregnancy in a post in December following months of speculation. She said at the time that becoming a mom was her "greatest dream realized."
"We are having a baby!" the television personality wrote. "I had been waiting and wondering but God had a plan all along."
"I still at times can't believe that our love created life!" she gushed.
"Tristan, thank you for loving me the way that you do! Thank you for treating me like a Queen! Thank you for making me feel beautiful at all stages! Tristan, most of all, Thank you for making me a MOMMY!!!"
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