The 31-year-old catwalk queen, who already has a two-year-old son Benjamin, is three months pregnant, reported Us magazine.
"Yes, she is three months pregnant. They are really happy!" a source said.
Bundchen and Brady, 34, got married in April 2010. The couple also raise 4-year-old Jack, who is Brady's son with ex-girlfriend Bridget Moynahan.
Bundchen previously said that she wanted a big family.
"I want more kids for sure, but I don't know when.I have my hands full (with Benjamin and her stepson Jack).
"For Benjamin, his big brother is his hero. Jack comes in and Benjamin has a big smile. He wants to follow him around. Anything he does, he's just in awe of him like, 'Oh my God!'," she said.