The 30-year-old Dutchess of Cambridge and her husband have been reportedly trying to conceive ever since they were married 16 months ago.
Sources have said that the couple is "delirious with joy" over finding out that they are expecting a child.
"Friends say Kate is convinced it's happened, and while she and William are keeping the news close to the vest, they're delirious with joy," a source told Star magazine.
"A woman can always tell when the earliest signs appear, and it's believed that a passionate night together at their rented country house on an estate in Wales, just after the end of the London Olympics, did the trick," the "insider" claimed.
If Middleton is actually with child then she'd be in early stages of pregnancy and only make a formal announcement when she is further along. Couples typically wait until they are about three months pregnant before announcing that they are expecting.
"You won't get anything out of me. Tight lipped," Prince William said when he spoke to journalist Kate Couric. "I'm just very keen to have a family and both Catherine and I, you know, are looking forward to having a family in the future."
Kate Middleton and Prince William were together for 10 years before they married. Their high-profile ceremony took place on April 29, 2011 at Westminster Abbey.
According to the Christian Post, Prince William felt pressured to have children.
"I'm still trying to decide. It's a really difficult one because I really enjoy my time in the Air Force and I'd love to continue it but the pressures of my other life are building. And fighting them off, or balancing the two of them, has proven quite difficult," Prince William reportedly said. "More importantly, I'd rather like to have children. So that's the key thing really."
Hollywood Life reported that Middleton is hoping she is having a girl.
If Middleton is pregnant, do you think she would name a baby girl Diana?