Talking through tears, a California woman held captive for nearly two decades told of the pain and determination as she gave birth to her captor's child in his backyard prison, while she was still just a young teenager.

"It was very painful," Jaycee Dugard told ABC News' Diane Sawyer in an interview on "Primetime" that aired Sunday night. "She came out and then I saw her. She was beautiful. I felt like I wasn't alone anymore. I had somebody who was mine."

The 31-year-old woman, usually clear and composed, grew emotional when she talked about seeing the first of two girls fathered by her kidnapper, Phillip Garrido.

When Sawyer asked how old she was at the time of the birth in the San Francisco Bay Area city of Antioch she said "14" with a small, incredulous laugh and a shake of her head.

She said she didn't know how she could protect the child, but said "I knew I could never let anything happen to her. I didn't know how I was going to do that, but I did."

Dugard talked to Sawyer on a couch and on a porch at her California home. The blond hair she had in now-familiar photographs from her childhood is now reddish-brown, and she wore a red sweater and a necklace with a pinecone charm on it, representing the last thing she touched before her 18-year captivity.

The interview came on the eve of Dugard's memoir about her time in captivity, "A Stolen Life," which will be released Tuesday.

Dugard told Sawyer there was "a switch" she had to shut off to emotionally survive her rape and imprisonment. Asked by Sawyer how she stayed sane, Dugard said: "I don't know. I can't imagine being beaten to death, and you can't imagine being kidnapped and raped. You just do what you have to do to survive."

She described walking to the school bus stop on the day of a fifth-grade field trip and being zapped with a stun gun on a South Lake Tahoe street at age 11.