Caitlyn Jenner's family tree is expanding. The 66-year-old former Olympian's oldest son, Burt Jenner is expecting a child with his longtime partner Valerie Pitalo. Burt Jenner, a 37-year-old race car driver, posted the exciting news on Thursday to his Instagram account posting a sonogram photo of his unborn baby boy.

"I promised to wait until we had the 'all healthy' update and we got it yesterday... My Lil boy, just chillin..." he captioned the post. "So happy I can't see straight, he can't get here fast enough... Who wants to fab me a stroller with @kingshocks and @gladiatortires like you see at the 1000 every year?"

Burt Jenner did not reveal the couple's due date, but shared that he is already exciting about bonding with his son over racing cars and bikes. He posted the hashtag, "#HesSponsoredAndHeDontEvenKnowIt." Pitalo posted the same sonogram photo to her own Instagram account.

"Oh boy!" she captioned the photo with a series of Christmas emojis.

Caitlyn Jenner has not mentioned his son's joyous news on social media. In July, the reality star's son Brandon Jenner and his wife Leah welcomed their first child together, a daughter they named Eva James.

"On Wednesday, July 22nd I became a Mom," Leah wrote on Instagram. "It has without a doubt been the most profoundly beautiful time in my life but also emotionally, spiritually and physically challenging too....and I am so grateful for every second of it. Brandon and I are taking care of our healthy, little baby girl, Eva James Jenner."

A photo posted by Burt Jenner (@bjenner) on Dec 24, 2015 at 9:05am PST