The 30-year-old model and 36-year-old singer announced the news on social media following the release of Teigen's Vogue Thailand cover.
"So proud and honored to have shot the cover of Vogue Thailand! Special to me for so many reasons...one being I am SO PROUD to be Thai, so excited to have shot a Vogue cover (a dream, of course) annnnd a couple minutes after this shot, John and I learned we were having our little baby girl!" Teigen wrote.
Teigen and Legend met on the set of the singer's "Stereo" music video in 2007, and married in September 2013. The couple revealed they are expecting their first child in October, just one month after Teigen shared her fertility struggles on FABLife.
"We would have kids five, six years ago if it'd happened, but my gosh, it's been a process," she said.
Legend recently told Us Weekly he and Teigen are considering both "traditional" and "creative" names for their firstborn. He admitted the model "gets a little self-conscious" about her growing body, but said he's "excited to be with a beautiful, pregnant woman."
Teigen is known for her modeling work with Sports Illustrated, and hosts Spike TV series Lip Sync Battle, which will return for second season Jan. 7. Legend last released Love in the Future in August 2013, which includes the hit single "All of Me."