Jessica Simpson Drinks Water, Not Alcohol, on Saturday Night

Singer Jessica Simpson arrives at the Us Weekly Hot Hollywood party held at Eden on April 26, 2011 in Hollywood, California.  Jason Merritt/Getty Images

No more margaritas for Jessica Simpson?

On Saturday, the new mom to daughter Maxwell Drew Johnson announced to her 5.4 Twitter followers that she was laying off booze.

"Responsibility has drastically changed my life!" Simpson, 31, wrote. "Saturday night drink of choice…. WATER! :)"

Breastfeeding little Maxwell, now 3 months, Simpson (a well-known fan of the margaritas at high-end Mexican chain Dos Caminos) has more than one reason to eschew too much booze: Having inked a deal with Weight Watchers, she has until the end of August to lose 20-30 pounds of the estimated 70 she gained while pregnant with her first child.

"Jessica has always had issues with junk food," an insider told Us Weekly of the star, who's training again with celeb coach Harley Pasternak.

The singer and fashion designer stepped out last week in L.A. with her family, and covered up her curves in a brightly colored printed caftan, and wore sky-high platform heels.

"[Weight Watchers] focuses on healthy habits for the long-term (and I can still indulge in my guilty pleasures every now and then, too)," the new mom explained on Facebook. "I have actually gotten a group of friends together who are going to be doing it with me."

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