Four years after their 2000 wedding, Brad, 54, fell for Angelina Jolie, 43, on the set of Mr & Mrs Smith.
"Brad and Jen had their share of problems when they were married the first time, especially when it came to having children," an insider tells In Touch.
"As far as the baby is concerned, from what I've heard, Jen got pregnant naturally with Brad. It just happened - no IVF or anything," says the insider.
"It's a true miracle baby," adds the source.
In Touch further adds that Jen and Brad first shared the news with Brad's parents, Bill and Jane.
As New Idea previously reported, the Pitts were the ones who urged Brad to explore a reunion with Jen after her split from Justin Theroux.
"Jane's always had a special kinship with Jen and was instrumental in getting back together with Brad," says the insider.
"Jen stayed in touch with Jane after she and Brad divorced 13 years ago," and Jane always preferred Jen, who's warm and down-to-earth, compared to dramatic Ange.
"They were over the moon about the news! Jane truly considered it a blessing."
The other people who are overjoyed with the new arrival is Brad's other children - Maddox, 17, Pax, 14, Zahara, 13, Shiloh, 12 and twins Knox and Vivienne, 10 - adds In Touch's report.
"As far as his kids are concerned, the bigger the family, the better."
However, In Touch further claims that Angelina has still not caught wind of the news.
"She was already livid that Brad was romantically involved with her arch rival," a source says, adding, "She's going to explode when she's hit with the news of Jen's pregnancy."
Brad Pitt and Jennifer Aniston are yet to respond to In Touch's report.