The 20-year-old reality star is believed to be expecting her first child with her boyfriend Travis Scott, and Spencer - who has known the famous family since starring on 'The Hills' with Kylie's half-brother Brody Jenner - has praised her maternal instincts.
Spencer - who is expecting his own child with his wife Heidi Montag - says Kylie used to ''love playing mum'' when caring for her ex-boyfriend Tyga's four-year-old son King Cairo.
He said: ''I used to watch her with the babies and King, she loves playing mum and cooking all the meals - I think she'll be an amazing mum. Kylie grew up faster than any other person - she may only be twenty but she grew up so fast she may as well be 30.''
And despite rumours of the 'Life of Kylie' star's pregnancy only surfacing last week, Spencer, 34, believes the lip kit mogul told her close circle of friends ''six weeks ago.:
He added to The Sun Online: ''She started telling people she was pregnant six weeks ago, because the person I know who heard the news, knew [she was pregnant] back then.''
Meanwhile, the 'Keeping Up with the Kardashians' star's famous family is said to be ''over the moon'' at the news of a new arrival.
A source said: ''The family is over the moon! Kylie loves how close knit her family is and is thrilled to bring that to her immediate family. Kylie feels more prepared than ever to be a mom.''
However, another insider claims Kylie's family are worried she hasn't thought it through in its entirety.
They said: ''Kylie is very naive. Spending time with other people's kids is obviously completely different than being a mom 24/7. Kylie of course has no idea about all the hard work and sleepless nights.''
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Sometimes actors are not acting.