Credit: Kevin Mazur/WireImage for Jessica Simpson Collection

Jessica Simpson won't give birth until next spring, but she's already planning her postbaby slimdown!

The mom-to-be, 31, is in talks with Weight Watchers for a $4 million deal to shed pregnancy pounds, Us Weekly has exclusively learned.

Simpson -- engaged to ex-NFL star Eric Johnson -- had initially intended to sign with the company to lose weight she had gained over the past few years.

"But then she got pregnant," says the source. "So this was the perfect compromise."

Under the contract, Simpson would have one year to use the point-counting program to "lose a signficant amount of weight," says the insider.

She's then show off her results in ads for the company -- much like current spokeswoman Jennifer Hudson.

Despite the deal, Simpson isn't letting her weight gain get her down.  "I'm bigger than I've ever been," she recently told Us. "But I want to show off my bump!"