The new parents made the announcement on Sunday. Teigen posted a screenshot of their little girl's birth weight -- 6 lb., 11 oz. -- along with the caption, "She's here! Luna Simone Stephens, we are so in love with you! And sleepy. Very sleepy."
The exciting news comes just one month after Kim Kardashian threw the Sports Illustrated model an untraditional baby shower at her home in Los Angeles, complete with a dessert table, McDonald's catering and customized Snapchat filters.
"My everything," she wrote. "Excited to meet this little girl soon and to see you shower her with the same love you give to me! We are so grateful to be surrounded by so many loving, kind, hilarious, generous, thoughtful beings. Feeling so lucky, so happy and so ready!"
The two enjoyed a much more formal baby shower on March 6 in New York City with their closest friends and family. At the fancy fête, Teigen donned a bejewled "baby mama" crown emblazoned in glitter and guests enjoyed cookies shaped like baby carriages, bottles and onesies.
As ET previously reported, the new parents specifically chose their baby's gender when Teigen underwent in vitro fertilization last December, two months after they broke the news that they were expecting via Instagram.
"I've made this decision," Teigen later told People. "Not only am I having a girl, but I picked the girl from her little embryo. I picked her and was like, 'Let's put in the girl.'"
"I think I was most excited and allured by the fact that John would be the best father to a little girl," she continued. "That excited me. It excited me to see … just the thought of seeing him with a little girl. I think he deserves a little girl. I think he deserves that bond."
"As many of you know, we've been trying to have a baby for a while now," Teigen captioned an Instagram photo. "It hasn't been easy, but we kept trying because we can't wait to bring our first child into the world and grow our family."
"We're so excited that it's finally happening," she continued. "Thank you for all your love and well wishes. I look forward to all the belly touching! Xx."
Teigen and Legend, 37, tied the knot in Lake Como, Italy, in September 2013. The "All of Me" singer proposed to Teigen in December 2011 after four years of dating. The two are also "parents" to three furry friends -- Puddy, Pippa and Penny.