As mum-to-be of the heir to the British throne, the Duchess of Cambridge’s pregnancy has played out in the most public way. But despite the world’s eyes being trained firmly on her growing bump, she and husband Prince William were able to share a delicious secret no-one else could possibly know. That was, until Kate’s adorable slip of the tongue last week, when it appeared she accidentally confirmed they were having a baby girl.

Sandra Cook, among the throng of 2000 fans who greeted Kate in the British town of Grimsby, was right next to Kate when she apparently let the cat out of the bag. “The lady next to me gave her a teddy bear and I distinctly heard her say, ‘Thank you, I’ll take that for my d…” Then she stopped herself,” Sandra explains to the Daily Mail. “I leaned over to her and said, ‘You were going to say daughter, weren’t you?’”The usually cool-under-fire Kate was visibly rattled, and attempted to deflect her gaffe with a laugh. But it was too late. “She replied, ‘No! We don’t know!’” says Sandra. “I said, ‘I think you do!’”

Then Kate appeared to contradict her insistence that she didn’t know the gender, exclaiming to Sandra, “We’re not telling!” But Sandra is convinced. “I’m certain it’s a girl. She wouldn’t have said the D-word otherwise. After she said it, she was a little flustered.” It’s not the first time Kate has accidentally revealed a family secret. Despite refusing to release the name of their puppy, Lupo, weeks later she spontaneously revealed the pooch’s name to a bunch of schoolkids.

While it’s palace protocol to keep the sex of royal offspring a secret until their birth, Kate’s awkward denial has been taken globally as confirmation that a little princess is on the way. Even if the secondborn is a son, a new law before the British parliament means if  Wills’ firstborn is a girl, she will one day be Queen.

Read more about Kate and William's baby news in this week's Woman’s Day on sale Monday March 11, 2013.