The model and TV presenter was updating fans on her pregnancy when she let slip the sex of the baby.
Teigen, 34, referenced the child as “he,” saying: “It’s so weird because the baby’s really, really healthy. He’s big. He’s probably – oops. That was stupid.”
Realising she had said more than she intended, Teigen covered her mouth with her hand before continuing to discuss her pregnancy.
She shared a clip of the slip on social media and captioned it: “Hahahahaha might as well tell you.”
Teigen is expecting her third child with Grammy-winning singer Legend, 41. The couple are already parents to daughter Luna, four, and son Miles, two.
In her latest video, Teigen discussed her previous struggles while pregnant, telling fans “my placenta sucks”.
She said: “It’s always been kind of the bad part of my pregnancies with Luna, with Miles. It just stopped feeding him. It stopped taking care of him.
“I was stealing all his food because I was getting huge but he wasn’t getting big at all, so he had to come out early and Luna had to come out early. I was induced both times.”
Teigen, who said doctors have put her on “serious bed rest,” said the baby is “growing beautifully.”
“Everything’s good,” she added. “I’m feeling really good, but my placenta’s really, really weak and it’s causing me to really bleed a lot. So basically, it’s just pretty high risk.
“This poor thing has been through so much already. We just have to get my placenta healthy again, and that means not moving so I’m on complete and total ‘don’t get out except to pee-pee’ bed rest.”
Teigen and Legend confirmed they were expecting a third child in August, announcing the news in the music video for Legend’s single Wild.
Reporting by Press Association