The Oscar-winner's publicist said she and her husband, Jim Toth, are thrilled, adding: "Both mom and baby are healthy".
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The 36-year-old Legally Blonde and Walk The Line star grew up in Nashville, Tennessee.
And she's very proud of her "definitive Southern upbringing" which, she says, has given her "a sense of family and tradition" and taught her about "being conscientious about people's feelings, being polite, being responsible and never taking for granted what you have in your life."
Reese is already mum to Ava, 13, and Deacon, 8, from her previous marriage to Ryan Phillippe.
After they split up in October 2007, she dated Brokeback Mountain star Jake Gyllenhaal, before finding lasting love with Jim, a talent agent.
Their fairytale wedding in the presence of friends such as Robert Downey Jr, Scarlett Johansson and Tobey Maguire was featured exclusively in HELLO!