Teigen, 34, accidentally announced the sex of her unborn third child with Legend, 41, while discussing her "high-risk" pregnancy Thursday on Instagram Stories.
Teigen said she is experiencing heavy bleeding that has affected the health of her placenta.
"My placenta sucks. It's always kind of been the bad part of my pregnancies," she said. "It's super weak. It's so weird, because the baby's really, really healthy. He's big. He's probably getting... oops! I'm stupid."
Teigen reposted the video on Instagram after accidentally sharing the news. She told fans she is on "complete and total bedrest" in the hopes of improving her placenta's health.
"Basically, it's just pretty high-risk and this poor thing has been through so much already," Teigen said of her unborn son. "So we just have to get my placenta healthy again, and that means not moving."
"Thank you for all your well-wishes, and thank you for all the donuts," she added.
Teigen and Legend married in September 2013 and already have two children, daughter Luna, 4, and son Miles, 2. The couple announced in Legend's "Wild" music video in August that they are expecting their third child.
Legend later said on Today that Teigen's pregnancy was a "quarantine surprise" amid the coronavirus (COVID-19) pandemic.
"We're very excited. It was a surprise -- a little quarantine surprise, I guess you could say," he said. "We're very excited, and we're grateful for all the well wishes we've gotten from people around the world."
Teigen shared a new video of Luna and Miles while celebrating her seventh wedding anniversary with Legend this week.
"Happy 7th anniversary! 14 years together! @johnlegend love u buddy. My best pal. You're like a brother to me!!" she joked in the caption.
Legend cooked Teigen a Spanish meal to celebrate the occasion.
"I love this woman so much and I love taking care of her, especially while she's growing our little baby. I'm so grateful for our love and our life together," he wrote on Instagram.