The baby will be Monica's first with professional basketball player Shannon Brown, whom she married in 2010.
"5 months & counting," Monica tweeted in response to Niecy Nash's weekend "Happy Mother's Day" message to her. "Excited."
The 32-year-old R&B singer didn't reveal her baby's gender. She has two sons -- ages 7 and 5 -- with her ex-fiance, rapper Rodney "Rocko" Hill, UsMagazine.com said.
Brown, 27, has a son from a previous relationship, as well.