The baby will be the first child for Khloé. Thompson, 26, welcomed a son named Prince Oliver with his ex-girlfriend Jordan Craig in December 2016.
“Yes, Khloe and Tristan are expecting and they are absolutely thrilled,” a source tells PEOPLE. “This isn’t something they were in a rush to announce to the whole world, and it’s only been in just the past week or so that they’ve felt comfortable telling most of their inner circle.
Khloé is the third Kardashian-Jenner sibling to currently be expecting a child.
“Things were complicated a bit by Kylie’s announcement as they don’t want to be seen as intruding on that moment for her but it is now reaching a point where Khloe doesn’t want to be openly lying to people, and just staying quiet isn’t going to work anymore,” the source continues.
“This is something Khloe has wanted for years and years and yet it’s more than that: she didn’t want it unless she was in the right relationship for it. She never let it happen with Lamar, intentionally, and she never considered it with James or French or anyone else she has seen. But this is such a wonderful moment for her: not only is she going to be a mother, she is going to be raising a child with a man she truly loves. Everyone is over the moon for them both.”
Reps for Kardashian did not respond to request for comment.
According to another insider close to the family, Kylie and Khloé are due around the same time. “They are looking at it like a unifying experience for them as sisters,” says the insider. “Getting pregnant at the same time was totally unplanned but they are really happy about it.”
RELATED VIDEO: Khloé Kardashian Relishes Domestic “Family” Life With Love Tristan Thompson in Cleveland
News of Khloé’s first baby on the way comes months after she admitted on the show’s season 13 finale — following news that her uterus looked normal after an initial fertility scare leading her to cease taking birth control — that she and her boyfriend have “definitely [talked] about starting a family” together.
“He wants to have about five or six kids with me and that’s lovely,” said Kardashian, who initially visited the fertility specialist to see if she could serve as a potential surrogate for sister Kim. “We could start at one and we could grow from there. But now knowing I’m not on birth control is scary. It’s like a really big step.”
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Kardashian and Thompson first stepped out together in early September, and have been spending much of their time together since then in Cleveland. Aside from New Year’s Eve, the pair also celebrated Halloween, Thanksgiving and Christmas together.
“Happy New Year!!! May you always be surrounded by the love you are deserving of! May Gods blessings continue to rain on us all!” she captioned the snap of Thompson embracing her.
Though the couple isn’t engaged, Kardashian opened up to ES Magazine in April about the possibility of tying the knot with the sports star, admitting she would say yes if he popped the question — and is excited about the prospect of starting her own brood.
“I would love to have a family. We’ve talked about it,” she revealed. “He [already] is a father, and I know for a fact that he would be an impeccable father.”
Added the Revenge Body star at the time, “I definitely want to be a mom. But I don’t put the pressure on it. It’s not like, ‘The clock is ticking.’ I feel in my soul it will happen.”
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In the season 13 finale of Keeping Up with the Kardashians, Kardashian — who has been seen on the show and throughout her and her family’s social media posts as the fun “Aunt KoKo” to her multiple nieces and nephews — admitted she wasn’t sure what her immediate future would look like.
“I am so relieved to know that I am able to get pregnant,” she said in her sit-down. “Now I have so many other things to think about. Do I wanna have my own baby now? I don’t know. I’m at a crossroads.”
The duo “are insanely happy and in love,” a source close to Kardashian told PEOPLE exclusively in January. And in a January post on her website, the now-mom-to-be reflected on how peaceful she has felt.
“It’s such a great feeling that I have my happy back! It’s from a genuine place and the most peaceful place I’ve been in for longer than I can even remember,” she wrote.
“I think when you’re in love you definitely feel happy and healthy too,” Kardashian continued. “I like that my relationship is very normal and doesn’t need to be so publicized all the time.”
- With reporting by KATE COYNE, AILI NAHAS and CHARLOTTE TRIGGS