Jennifer Lopez got the party started early for her 40th birthday.

At midnight this morning, the singer-actress -- who turns 40 today -- celebrated with husband Marc Anthony and the cast and crew of The Back-Up Plan on the flick's New York City set.

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She was all smiles while toasting with champagne and blowing out the candles on the giant chocolate cake, which featured a huge photo of herself.

Lopez -- who donned an off-the-shoulder top -- also hugged crew members who presented her with a bouquet of white flowers.

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"Alex, make a toast!" Lopez is heard saying to her costar Alex O'Loughlin in a video of the party.

"Happy birthday. Welcome back, baby! To Jennifer!" he says, holding up a bottle of champagne.

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The romantic comedy (out January 2010) is Lopez's first movie since giving birth to twins Max and Emme in February 2008.