Bristol Palin, Sarah Palin

Sarah Palin tells Barbara Walters that she was "shocked" by daughter Bristol's pregnancy because she had no idea the then-17-year-old was sexually active, ABC News reports.

"That's why I was shocked," Palin tells Walters in a new interview, which ABC will air in five parts, beginning Tuesday on Good Morning America. "Truthfully, we were devastated."

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Palin famously announced her daughter's pregnancy days after accepting the Republican vice presidential nomination in September 2008. Bristol Palin gave birth to son Tripp that December.

Palin's Tuesday sit-down with Walters will continue on Nightline Nov. 17, Good Morning America on Nov. 18 and 20/20 on Nov. 20.