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Pop star CHRISTINA AGUILERA reveals her growing baby belly and more in the January issue of Marie Claire, on stands December 11. On the cover, the expecting mom poses wearing nothing but a cropped jacket, proudly showing off the belly she kept hidden for months. But the article inside is just as attention-grabbing, with the singer opening up about her "surprise" pregnancy.

"We were planning on starting to try after the tour," she says. "And so, I had gone off the Pill to prepare my body, because I didn't know how much time it would take. You've heard it takes some time -- except with Power Egg and Super Sperm here ... I'm like, 'Oh, my God, can you believe it just happened?'"

Christina says getting pregnant so quickly left her in a bit of a bind regarding the tour. She had a month left to go and each show consisted of 10 costume changes -- including major heels, hanging from a pinwheel and riding a carousel horse.

"I was paranoid. There are so many things that could go wrong -- somebody could slip, somebody could fall, I could fall," she says. "There was no way in hell I was going to jeopardize my baby for my show."

So she wore a very discreet heart monitor and she didn't announce the news. "I didn't want to make the audience uncomfortable," she says. "Like, 'Pregnant lady onstage! Is she going to be OK?'"