Kate Middleton and Prince William don't know the sex of baby No. 3 -- here's why

Kate Middleton and Prince William have remained mum on the sex of baby No. 3, and for good reason. It turns out the royal couple haven't found out the sex of their third child.

"William and Kate don't know the sex of the baby, so they're rather looking forward to finding out," a source told E! News. "It's very difficult to be totally prepared when you don't know if it's going to be a boy or a girl."

Kate and William also didn't know whether they were going to have a boy or girl with George and Charlotte either. Royal tradition forbids parents from finding out the sex of their child before the birth.

However, there have been some royals who have broken the tradition -- like Princess Diana who already knew she was going to have sons Prince William and Prince Harry. However, she didn't disclose the exciting news to the public until it was announced by Buckingham Palace after their births.

When Kate gives birth to baby No. 3, the announcement of the birth, the sex of the baby, and her well-being will probably be posted outside Buckingham Palace at one time. We say "probably" because when Princess Charlotte was born, Kate and William made the announcement on Twitter before Buckingham Palace released the official statement.

The Duchess of Cambridge is officially on maternity leave until fall, but it doesn't mean we won't be seeing her at all. All eyes will definitely be on Kate at Meghan Markle and Prince Harry's wedding on May 19.

Check out the slideshow above for all of Kate's most memorable looks during her third pregnancy.