The sources claim Khloe is three months pregnant and conceived naturally.
Khloe and Tristan, 26, reportedly have yet to learn whether they have a baby boy or girl on the way.
Khloe, however, just shared a photo of herself on Sunday in which she was posing in a bikini and sweatpants in front of a large mirror and showing off her tight abs.
Khloe and the NBA player began dating in September 2016 after allegedly meeting on a blind date set up by one of the reality TV star's friends.
When addressing engagement rumors, Khloe told You magazine last month, "I'm in the best relationship I've ever been in and it doesn't take a ring for me to feel that way."
"My boyfriend is very protective of me. Feeling loved and validated and secure is a huge thing," she continued in the interview. "Tristan is protective of my entire family, which is beautiful. And I love that everything we do is as a union. When he speaks, it's 'us' and 'we,' and that's important because it shows that you're sharing life."
"We're still in a new relationship and I love us having time together," she explained to You. "Once you have kids, you can't get back your non-kid years... You worry about your children for the rest of your life."
But in June, an insider expressed to People how Khloe "really wants a baby" and "hopes it will happen soon."
Khloe also revealed on the Season 13 finale of Keeping Up with the Kardashians at the time that Tristan "wants to have, like, five or six kids with me, and that's lovely. We could start at one and then grow from there. But now, knowing I'm not on birth control, it's really scary. It's like a really big step."
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Khloe, who lives in the Los Angeles area, has been making a long distance relationship with Tristan work, as he plays for the Cleveland Cavaliers in Ohio.
"She flies [to Cleveland] to see him almost every week," a source told Us of the romance. "Things are going really well. The relationship is very serious."
Tristan is already dad to a nine-month-old son named Prince, whom he welcomed with his ex-girlfriend Jordan Craig.
Khloe finalized her divorce from Lamar Odom, a former NBA star, in December 2016 after initially splitting in 2013.
The news of Khloe's pregnancy comes hot on the heels of multiple media outlets reporting that Kylie Jenner is also expecting her first child. Kylie is reportedly pregnant with boyfriend Travis Scott's baby.
Meanwhile, Kim Kardashian and husband Kanye West are anticipating the arrival of their third bundle of joy via surrogate. Kim and Kylie's babies are reportedly due around the same time, in mid-January.