Paris Hilton and fiance, Chris Zylka. Picture: Instagram.

American star Paris Hilton has admitted she has "baby fever" after spending time with her sister Nicky's newborn daughter Teddy.

The socialite's sister Nicky Hilton Rothschild and her husband James welcomed Teddy Marilyn into the world on December 20, and the proud auntie has revealed spending time with the baby girl has made her want a child of her own. 

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Also impressed with Nicky's post-baby figure, Paris - who is engaged to actor and model Chris Zylka - told Us Weekly: "You literally cannot tell ... [I've got] baby fever ... Also it gives me hope, because I know she has amazing genes, it runs in the family, so I'm not gonna have to worry." 

Nicky, 34, and her husband are also parents to 19-month-old daughter Lily Grace Victoria, and they revealed the birth of her sibling on social media late last year. 

Taking to Twitter to announce the happy news, Nicky wrote: "Christmas came early! On December 20th James and I welcomed a baby girl, Teddy Rothschild into the world. Feeling very blessed this holiday season. (sic)" 

Prior to the birth, Nicky revealed that she felt "very prepared" for the arrival of her second child. 

She shared: "I feel very, very prepared. I know what to expect. I'm good, I'm ready." 

Nicky also said that her eldest daughter was looking forward to meeting her sister. 

The reality star explained: "I think she'll be really sweet. She doesn't have a jealous bone in her body. 

"When I hold other babies, she doesn't care. She's very sweet. I try and explain it, [but] she's too young. I did get her a doll that she gives a bottle and takes in the bath with her."