William & Kate's Baby Joy!

Congratulations are on order! Sources are buzzing that Prince William and Duchess Kate are expecting their fourth child — and it’s a girl!

It was quite the day. On Aug. 8, Duchess Kate and her husband, Prince William, hosted and competed in the U.K.’s inaugural King’s Cup Regatta. “They both seemed to be in the best mood, especially Kate. She looked absolutely incredible and was positively glowing,” says an eyewitness, adding that onlookers were surprised to see that the normally pin-thin royal looked much healthier — and was noticeably protective of her stomach area, especially when putting on her sailing harness. “People couldn’t help but wonder if Kate is pregnant again.”

It seems that they were onto something. “William and Kate are expecting their fourth child,” a palace insider tells OK!, adding that the Duchess of Cambridge, 37, is rumored to be around three months along — and unlike with past pregnancies, she’s opted to find out the gender. “It’s a girl!” the insider continues. “With their first three children [George, 6, Charlotte, 4, and Louis, 16 months], Kate and Wills wanted it to be a surprise. But this time, they thought it would be fun to just enjoy the planning process — decorating the nursery and picking out adorable clothes and toys. So they decided to buck tradition and learn the sex.” It’s a decision they don’t regret. “William and Kate were so excited when they found out they were having another daughter,” adds the insider. “They’ve always said they wanted four kids — two boys and two girls — and feel so grateful that their dream has come true.”