During an Instagram Story, Teigen made the mistake when she was speaking candidly about her pregnancy struggles.
"My placenta sucks, It's always been kind of the bad part of my pregnancies, with Luna, with Miles, it just stopped, it stopped taking care of him... I was getting huge, but he wasn't getting big at all," she explained.
She let the gender slip when she was letting her followers know that the baby was doing well.
"The baby is really, really healthy. He's big — oops," she stopped, realising she had just revealed that she was expecting a baby boy.
But the 34-year-old took the slip in stride, captioning the video "Hahahaha might as well tell you".
"So [he's] growing beautifully. I'm feeling really good but my placenta's really, really well, and it's causing me to bleed a lot. So basically, it's just pretty high risk and it's just, this poor thing has been through so much already," she said.
"We just have to get my placenta healthy again, and that means not moving much. So I'm complete and total don't-get-out-except-to-pee bed rest. It's such a bummer because I had to cancel so much work, and I was so excited to talk to so many different people and do these events, because it would start to feel like normal life again. The timing is just shit, I apologise, but I have to make my baby okay, so there you go. Thank you for all your well-wishes."
The baby boy will be a brother to Luna, four, and Miles, two.