Jessica Simpson knows how to make the most of a cheat day.

The Weight Watchers spokeswoman -- expecting baby no. 2 -- went all-out at a December 2 birthday lunch for BFF CaCee Cobb, 35.

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"Jessica ordered a grilled cheese sandwich with onion rings," a fellow diner at the Polo Lounge in Beverly Hills tells Us Weekly.

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As her friends toasted Cobb with cocktails, the mom to Maxwell, 7 months -- who has yet to confirm her pregnancy -- waved off her usual white sangria.

"Guess what?" she told her server, with a smile. "I'm drinking ginger ale!"

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In anticipation of the baby's arrival, Simpson, 32, and fiance Eric Johnson, 33, are in nesting mode: A source says they just made an offer on a house in L.A.'s luxe Hidden Hills!

This article originally appeared on Pregnant Jessica Simpson Eats Grilled Cheese, Onion Rings, Avoids Alcohol at Pal's Birthday Bash