Graham already shares a baby boy called Isaac, who she welcomed in January, with her videographer husband Justin Ervin.
She told her 13.3 million Instagram followers: “The past year has been full of tiny surprises, big griefs, familiar beginnings and new stories.
“I’m just beginning to process and celebrate what this next chapter means for us.”
Her post had attracted more than 188,000 likes within half an hour.
Since having her first child earlier this year, the catwalk star and body confidence activist has regularly updated her followers on her first steps as a new mother.
Graham shared a picture of the stretchmarks she developed while pregnant with Isaac, and she also revealed that she had to change Isaac’s nappy while out shopping “with no restroom in sight”.
She spent part of lockdown self-isolating with her mother and husband in her native Nebraska.