Star adds that both Prince William and Kate are "thrilled" to welcome another child, they're really hoping for a girl to join their brood.
"All they really care about is having a healthy baby," says the source.
However, despite the happy news, friends and family of the Duchess are frightened her morning sickness could return - worse than ever!
"Most people gain weight during their pregnancy, but Kate could actually lose weight because of her condition," an insider tells Star.
"She has to be extra cautious. It must be tempting to skip meals if she's feeling too nauseous to eat, but apparently she's doing her best to keep it under control."
Star adds that the news also couldn't have come at a better time for Kate, who "loves the idea of her and Meghan having children that are the same age."
"They're already excited for all the play dates."
Despite wanting to share the news, Star claims Kate is being careful not to steal Meghan's thunder.
"It's no surprise that Kate isn't shouting from the rooftops," Star's insider notes, "She knows how excited Harry and Meghan are, and she wants them to enjoy this time."
Queen Elizabeth II is another royal family member who is thrilled with the news!
"The Queen has four children, so this would be a nice way for Kate to follow in her footsteps."
The Royal Family is yet to respond to Star's report.