Jamie-Lynn Spears, the 16-year-old sister of Britney Spears tells OK! magazine she's pregnant - Britney Spears didn't even know.

Britney Spears' sister Jamie Lynn is pregnant at the age of just 16.

Jamie Lynn has revealed she is "shocked" to be expecting her first child with boyfriend Casey Aldridge.

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

The 'Zoey 101' star, who is 12 weeks pregnant, kept the news a secret until she got used to the idea. She told her mother Lynne shortly before Thanksgiving.

She told America's OK! magazine: "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.

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

Jamie and Casey, who have known each other for years, began dating in high school.

Although Jamie is pleased about the pregnancy, she has advised her young fans to abstain from sex until they are married.

She said: "I definitely don't think it's something you should do, it's better to wait. But I can't be judgmental because it's a position I put myself in."

Lynne - who has had to postpone the release of her parenting book about raising her famous daughters indefinitely - admitted: "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."

Britney, 26 - who only has supervised visits with her sons, two-year-old Sean Preston and 15-month-old Jayden James after losing custody to ex-husband Kevin Federline - did not know her sister was pregnant.

The 'Gimme More' singer was shopping late last night when she was told the news by a photographer.

She replied: "My sister's not pregnant!"

A source close to the star said: "This is going to kill her. In the end, she'll be supportive, but this is a big shock.

"Britney doesn't have to listen to anyone now but her sister still does so who knows what people are telling her. She has her career now too."

Dlisted

TMZ's scoop:

TMZ spoke exclusively with Joyce Aldridge, mother of Casey, who confirmed that Jamie Lynn Spears is indeed pregnant with her grandchild.

Joyce said, "We are aware of the recent interview regarding her and being pregnant, and we are in agreement with everything that was said by Jamie Lynn about the situation ... everything is fine."

When asked if Casey had anything to say about his girlfriend being preggo, she said, "At this point, Casey doesn't wish to speak out, but it would be wonderful when the time comes." When asked about their ages and any negative aspects, she said, "Casey will address all of this when it is time."

TMZ also spoke with Casey's grandfather, James, who did not know about the pregnancy, but confirmed his grandson is dating Britney's sister, and that his son Mark, Casey's dad, works at a paper mill in Cleveland, TN.