Britney's little sis is expecting, according to OK! magazine.

The Spears family is going to be growing soon, but it's not BRITNEY who has confirmed her pregnancy -- it's her 16-year-old little sister, actress JAMIE LYNN SPEARS, who is going to be a mom, OK! magazine is reporting. A rep for Jamie Lynn has not returned calls from ET at this time.

In the OK! interview, Jamie Lynn says that the father of her unborn child is her long term boyfriend, CASEY ALDRIDGE: "It was a shock for both of us, so unexpected. I was in complete and total shock and so was he."

According to OK!, Spears is 12 weeks along and told the mag that she waited until two weeks after she found out she was pregnant to tell anyone. She told her mother just before Thanksgiving.

Jamie Lynn says her mother, LYNNE SPEARS, was "very upset" at first, but came around after a week and "became very supportive."

OK! reports that Lynne says she was also shocked by the news: "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."

Nickelodeon, the cable network that Jamie Lynn's shows, "Zoey 101," airs on, released this statement late Tuesday, "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."

Please keep checking back here for details on this breaking story as they develop.