Portman is engaged to the father of her baby, actor/choreographer Benjamin Millepied. The couple met while working on "Black Swan."
"They've only told a very small group of family members and friends that they're expecting a son," says a source close to the couple. "They are so excited. Benjamin cannot wait to be a father."
Natalie is due in early spring.
For more expectant celebs, check out "Extra's" Baby Bump Watch 2011.