Credit: David McNew/Getty Images; Gilbert Carrasquillo/FilmMagic

In an upcoming interview with Barbara Walters, former GOP vice-presidential contender Sarah Palin says that she did not know her daughter Bristol was sexually active with Levi Johnston.

So when Bristol, then 17, told her mom she was pregnant, "I was shocked," Palin, 45, tells Walters in the five-part series of interviews to begin airing on ABC's Good Morning America Tuesday.  

"Truthfully," Palin adds, "we were devastated."

Bristol, now 18, gave birth to son Tripp in December. She and Johnston, 19, split shortly after.

Johnston, who posed in Playgirl this past week, said he plans to file for joint custody of Tripp because the Palin family won't let him see the baby.

In her Oprah Winfrey Show interview airing Monday, Palin said Johnston is still "part of the family" and is even welcome over for Thanksgiving.

But during his Playgirl shoot, "He said ' ... she was full of it,'" Daniel Nardicio, Playgirl's marketing director, tells