The actress gave birth to the baby in Los Angeles on Wednesday, Moynahan's publicist Christina Papadapoulos said in a statement.
"Both mother and baby are doing well," she said.
Other details were not immediately released.
Moynahan and Brady split up late last year after a three-year relationship. Brady, who has been dating supermodel Gisele Bundchen, wasn't on the field or sidelines for the first 15 minutes of Patriots practice Wednesday.
The quarterback had said he wanted a few days off to attend the birth of his first child. It was unknown if he arrived before the baby was born.
"I hope so," Brady said Monday in regards to getting time off for the impening birth.
"Bill (Belichick) has a lot to say," Brady said, "so we'll deal with it when it happens."
New England plays the third of its four exhibition games -- the one in which Brady usually gets the most playing time -- on Friday night at Carolina. The first regular season game is set for Sept. 9 at the New York Jets.
It is unclear whether or not Brady will be available to play on Friday.
Copyright 2007 by The Associated Press