"Kate's been carrying this baby secret for a few weeks now," the palace insider tells Star, and notes they've put off the official announcement so as not to steal the thunder from Prince Harry and his pregnant wife Meghan, who are expecting their first.
The surprise couldn't have come at a better time for the couple. who "love the idea" of having children who are the same age, says Star's insider, noting that Kate is relieved to have a sisterly pal to turn to during the pregnancy.
In fact, the royal sister-in-laws have been brought closer together.
Kate and William can't wait to share the joyful news with the world.
"This is a precious time for them," a source says. "They feel so blessed."
The Royal Family is yet to respond to Star's reports.