Kelly Rowland regrets letting slip the sex of Beyoncé first baby.

While Beyoncé was pregnant with her and Jay-Z's daughter Blue Ivy Carter, her Destiny's Child bandmate Kelly slipped up in an interview and referred to their impending arrival as "she" several times.

Kelly owned up to the blunder during an appearance on Billy Mann's Yeah, I F**ked That Up podcast when she was asked to name the biggest mistake she'd made in an interview.

"Oh my God. When I made a mistake and told the sex of (Beyoncé)'s baby when she was pregnant with Blue," Kelly responded. "That was the worst moment ever. The worst moment ever."

The 42-year-old admitted she didn't even realise what she had done until she saw the headlines about her comments later.

"I was like, 'What are they talking about? I didn't say... Oh my God!'" Kelly recalled. "It was bad because it was no one's business."

Noting that her close childhood friend was "disappointed" by the reveal, Kelly added, "I felt terrible because it's not my news. It was honestly the worst. It wasn't my news to share, and I didn't mean it like that. The 'she' just kind of slipped out."

Beyoncé and Jay-Z welcomed Blue Ivy, now 11, in January 2012. They also share six-year-old twins Sir and Rumi.