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.
Celebrity Breakups and divorces of 2014 [PHOTOS]