Performing at the VMA awards in 2001. Photo by: GARY HERSHORN / REUTERS / CORBIS
The Two Sides of Britney| Britney Spears
Eager as she is to return to performing, she is not yet the Britney who could famously match her back-up dancers move for move. Although she is rapidly returning to her pre-baby body thanks to "a combination of cardio, weights, yoga and pilates and long walks with the baby," she is still troubled by the 2004 knee injury she sustained while shooting her "Outrageous" video. "It's fine if I go in the gym and start dancing, but when I really start challenging myself, it starts snapping," she says. "I just have a feeling if I really go for it, it's not going to happen."

She has changed in other major ways as well. The star who let it all hang out on last summer's reality series Britney & Kevin: Chaotic is now far more protective of her personal life. "I would love to talk about my pride and joy," she says of her son, "but once you do it with one interview, then the door is open and people take advantage of it." When asked if she could open the door to talking about her husband, she says, "just a little" before addressing his forays into hip-hop. "He goes into the studio and has his own vibe," she says. "I'm really impressed."

Clearly she has been burned by the endless scrutiny of her marriage. "Look at Jennifer Aniston – she went on Oprah just to say she's okay," says Spears. "It does affect you to a certain extent. It has to. I don't take myself so seriously, but when you've been poked at and poked at, there's going to come a time when you're going to snap."

Late in the interview, Spears's face lights up during a call to Federline to arrange their evening plans; the pair exchange easy laughs about Sean P. It is that little guy, after all, who has changed the singer's world for good. And she is keenly aware that juggling her fierce ambitions with the sometimes messy contradictions of motherhood – that unique mix of vulnerability and invincibility – will test her like nothing else. "In the car on the way here, I was crying because I was listening to country music," she says. "I love L.A., but I know what I am. I'm a country girl at heart. I long to be at home. I like that simplicity for my child. I really, really do."

• By Michelle Tauber. Michael Fleeman in L.A.