JESSICA SIMPSON owes her big weight-loss to a big secret – morning sickness!

Sources say that’s how the 5-foot-3 mom-to-be managed to shed the last 20 pounds of her 60-pound weight loss after giving birth to daughter Maxwell on May 1.

As The ENQUIRER reported last week, the 32-year-old “Fashion Star” guru is pregnant again – and, ironically, that’s why she’s finally managed to lose the last of her baby fat.

“Jessica really didn’t start showing a significant amount of weight loss until the last couple of months, after she became pregnant with her second child,” said a family insider. “She now believes that she conceived at the end of September!”

The reality TV bombshell weighed 100 pounds when she played sexpot Daisy Duke in the 2005 film, “Dukes of Hazzard" But Jessica gained more than 100 pounds while she was pregnant with Maxwell, her first child with fiance Eric Johnson.

As for her weight gain, “during preg­nancy, I didn’t really think about it,” she told talk-show host Katie Couric on “Katie” in early September. “But Jess was a little more chunky than she wanted to be when she ap­peared on ‘Katie,’” said the source.

By late September, Jess – who’d signed a $4 million contract with Weight Watchers – was down to 155 pounds but admitted she was still “eating for two” after giving birth.

Desperate to get her curves back, the singer-turned-actress considered having liposuction. She also reportedly went on an extreme four-month crash diet plan that featured low-fat smoothies, protein and plenty of vegetables.

But in the end, Jessica was able to shed the weight because she couldn’t keep her food down, sources said.

“Jessica was trying so hard to lose her baby weight that she never dreamed she could be pregnant again,” added a family friend. “But when her morning sickness kicked in, the pounds just melted away!

“She’s dropped more than 20 extra pounds and reached her goal  weight!”