Michelle Duggar proudly told NBC of the news on Tuesday - but her 19 other children got to hear the news first.
Duggar, 45, and her husband Jim Bob, 46, have more experience than most in new arrivals, but they said they were surprised by the latest.
"I was not thinking that God would give us another one, and we are just so grateful," said Duggar, star of a reality show on TLC called "19 kids and counting."
The family lives in Little Rock and also runs its own Internet site, which sells books written by the parents and DVDs, and dispenses advice to parents.
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The brood currently consists of ten boys and nine girls aged between 23 months and 23 years. The latest, Josie, was born prematurely in December 2009 and Duggar suffered complications.
Duggar said that despite the risks, she was not worried for her health.
"I'm really in better shape than I've been in 20 years," she said.
Amid the joy, Bob had a practical view on the situation.
"We didn't want to stop on an odd number," he said.
