Ambrosio and her fiance, California businessman Jamie Mazur, are already the parents of a 3-year-old daughter named Anja.
"Family is the most important thing for me and I'm thrilled to be adding to the family," Ambrosio told UsMagazine.com in a statement Wednesday. "I'm fortunate to have a loving and supportive family. It's a blessing that it will soon be growing."
The 30-year-old model told Brazil's Istoe Gente magazine she is four months pregnant, but doesn't know the baby's gender.
Ambrosio participated in last month's Victoria's Secret fashion show.