Teigen and Legend have been married for nearly 9 years and have two children, Luna and Miles.
The couple had previously said their first two children were conceived through in vitro fertilization and in her announcement on Wednesday, Teigen said the newborn was on the way “1 billion injections later.”
She said Teijan was reluctant to announce the pregnancy.
“On every date I said to myself, ‘Okay, if it’s healthy today, I’ll announce it,'” she wrote, “but then I sigh to hear the heartbeat and decide I’m still very nervous.”
She wrote, “Well bro, it’s been so hard to keep this up for so long!”
“Hardcore twitter fanatic. Proud coffee fanatic. Social media aficionado. Devoted tv enthusiast. Alcohol scholar. Bacon specialist. Avid troublemaker.”