Kim Kardashian tries not to smile for the cameras. (Instagram)

The 38-year-old reality TV star has told fans they can keep their skin looking youthful by resisting the temptation to smile whilst posing for a photograph.

Asked how she avoids getting wrinkles, Kim - who has kids North, five, Saint, three, and 12-month-old Chicago - told fans at an event in Los Angeles: "Don't Smile."

Kim - who is married to rap star Kanye West - also admitted that her first pregnancy and the reaction to her weight gain prompted a change in her attitude.

She explained: "It changed my mood; it changed who I was; it changed my personality a lot. It changed the way I viewed wanting my picture taken.

"After I had the baby, I was like, these are the same people that made fun of me ... I didn't want to smile for them."

The brunette beauty has also slammed rumours that she's had a nose job.

Instead, Kim insisted that becoming a mother simply led her features to "come out".

She said: "I never had my nose done.

"Pictures, I swear, I look at them and would be like, 'Wow the bump looks so much bigger in some pictures than in others.'

"Everyone thought I [had a nose job], and I said wait until I have kids because your real features come out."