After weeks of speculation, the Keeping Up With The Kardashians star confirmed the gender of her unborn child in a tweet to wish husband Kanye West a happy Father's Day.
She said: "You're such a good daddy to North & you will be the best daddy to our new son too!"
The 34-year-old also added emojis to represent a woman, man, girl and boy.
Kim and Kanye, 38, are already parents to daughter North, who celebrated her second birthday earlier this month. They confirmed in May that a second child was on the way.
As well as announcing the sex of their second child, reality star Kim also took time to celebrate Kanye's love for little North.
She tweeted alongside a picture of Kanye and North: "Your bond is so irreplaceable! Thank u for being such a good dad! We & appreciate u so much! So blessed to have u."
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