Another Trainwreck On The Way....

A source close to Britney Spears tells TMZ that the popwreck is "frantic" over the news that her baby sister, 16-year-old Jamie Lynn, is pregnant.

We're told that before the news hit today, Britney was not aware of her little sis's bun in the oven. Oops ... a Spears did it again.

Recently Britney has been estranged from her mother Lynne, but has spent time with Jamie Lynn. TMZ cameras caught the younger Spears defending her older sister from an angry neighbor last October. Those hormones are a bitch!

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.

TMZ