The reality star announced the news alongside an Instagram picture of herself holding little Reign's fingers.
She wrote: "Madly in love with my little Reign Aston Disick."
A source told E! News: "The family is freaking out with joy. What an amazing coincidence."
Earlier this month, Kourtney opened up to Fit Pregnancy magazine about expecting her third child.
She said: "I love being pregnant in general. This time I'm more tired than I was the other times.
"I think it's because I'm always running around and working on different things. But I feel comfortable with how I look and feel.
"I find a pregnant body to be beautiful and an incredible reminder of what a woman's body is made to do."