The 32-year-old and his 29-year-old wife announced the happy news on Tuesday via photos posted on their Instagram pages, which show him placing his hand over her baby bump. E! News recently obtained new and exclusive details about their upcoming arrival.
"Catherine is just out of her first trimester," a source said. "They've decided not to reveal the sex at this point."
"They want a calm, quiet pregnancy," the source added. "They're not interested in making a spectacle of it, which is why they did the announcement on their social media. That was how they wanted to tell the world."
Catherine is "glowing and feeling great" and has not suffered any pregnancy complications so far, the source told E! News, adding that both she and Sean are "so excited" to become parents.
"Sean is an uncle and has a lot of experience helping with his sister's kids," the source said. "Catherine knows he's more than ready to be a wonderful father."
The two met on the set of The Bachelor season 17 in 2013 and wed in a televised ceremony in January 2014. They made the pregnancy announcement days after the Dec. 4 premiere of WE tv's Marriage Bootcamp: Reality Stars season 4, which features the couple.
Earlier this year, Catherine upset several people when she posted a photo of herself sporting a fake baby bump as an April Fools' Day joke.
"It was intended to be lighthearted and fun and certainly not insensitive," she responded. "My heart goes out to all the women struggling with fertility."