"Kourtney has confessed she's expecting," said a source. "She's very excited and only telling a select few right now - but she just couldn't keep it quiet any longer."
"Kourtney has that kind of glow she had with Mason and Penelope," added the source. "They wanted to have one right after Penelope."
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Friends of the reality TV star and her model and businessman partner Scott have said the couple have been trying for another a baby since their one-year-old daughter arrived in July 2012.
During a recent trip to Miami Kourtney avoided drinking alcohol and eating raw fish which prompted speculation that she was pregnant.
The star, who has six siblings, has made no secret of the fact that she wants a large family.
"How many children do you think we could fit in this house?" Kourtney joked during an episode of Keeping up with the Kardashians. "In the playroom, I could build bunk beds for four."
"Children are such a blessing," Kourtney told HELLO! at the baby shower for her daughter, Penelope.
Speaking before her sister Kim Kardashian became pregnant Kourtney revealed that she wanted her sisters to have children too.
"It would be wonderful," said Kourtney four months before Kim announced that she and boyfriend Kanye West were expecting a baby.