Jamie Lynn Spears, Britney's teen sister, pregnant

Another Spears baby is reportedly on the way -- and it's not Britney's.

Jamie Lynn Spears, the 16-year-old "Zoey 101" star and sister of Britney, told OK! magazine that she's pregnant and that the father is her boyfriend, Casey Aldridge.

"It was a shock for both of us, so unexpected," she said. "I was in complete and total shock and so was he."

Spears is 12 weeks along and initially kept the news to herself when she learned of the pregnancy from an at-home test and subsequent doctor visit, she told the celebrity magazine, which hits stands in New York on Wednesday and the rest of the country by Friday.

What message does she want to send to other teens about premarital sex? "I definitely don't think it's something you should do; it's better to wait," she told the magazine. "But I can't be judgmental because it's a position I put myself in."

After she found out from a doctor that she was pregnant, she said, "I took two weeks to myself where I didn't tell anybody."

"Only one of my friends knew because I needed to work out what I would do for myself before I let anyone's opinion affect my decision. Then I told my parents and my friends. I was scared, but I had to do what was right for me," she said.

Spears broke the news to her mother, Lynne, just before Thanksgiving, the magazine reported.

"She was very upset because it wasn't what she expected at all," Spears said. "A week after, she had time to cope with it and became very supportive."

Lynne Spears, already grandmother to Britney's young sons, told the magazine: "I didn't believe it because Jamie Lynn's always been so conscientious. She's never late for her curfew. I was in shock. I mean, this is my 16-year-old baby."

She said her actress daughter, the telegenic heroine of her popular Nickelodeon series, has known Aldridge for years and began dating him in high school.

But in a recent interview with The Associated Press, Spears said she had no steady boyfriend. "I kind of just keep my options open," she said. "I have a bunch of friends that I always hang out with, a bunch of guy friends." She declined to talk about her older sister.

Spears spoke to the AP shortly before Thanksgiving, the day she told OK! she informed her mother about the pregnancy.

Jamie Lynn plans to raise the baby in her home state of Louisiana -- "so it can have a normal family life."

Nickelodeon released a statement: "We respect Jamie Lynn's decision to take responsibility in this sensitive and personal situation. We know this is a very difficult time for her and her family, and our primary concern right now is for Jamie Lynn's well being."

A publicist for the network did not immediately respond to phone and e-mail requests for additional comment.

"Zoey 101" is scheduled to conclude its third season Jan. 4 with a cliffhanger episode in which Spears' character must decide whether to leave her fictional Pacific Coast Academy boarding school in California to join her parents in London.

The show is scheduled to resume in February, and its producer, Dan Schneider, has said filming on the fourth season has already been completed.