She took to Instagram Wednesday to share photos of herself, her husband Mike Fisher and her son Isaiah with the newborn, who she said was born "in the wee hours of the morning on Monday."
"His mom, dad and big brother couldn't be happier for God to trust them with taking care of this little miracle," she wrote. "Our hearts are full, our eyes are tired and our lives are forever changed. Life is good..."
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The former "American Idol" winner revealed last year that she had three miscarriages in about two years.
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The Associated Press contributed to this report.