"Merry Christmas from my family to yours!" the singer and fashion entrepreneur posted on her Twitter page alongside a photo of daughter Maxwell Drew, nearly 8 months, with the words "Big Sis" written in sand before her.
Simpson's representative was not available for comment but had deflected an Us Weekly cover story last month that announced the pregnancy, telling CNN at the time: "I won't be commenting on speculation made by Us Weekly."
Simpson, 32, had confirmed her first pregnancy in similarly lighthearted manner, tweeting a photo of herself on Halloween 2011 dressed in costume mummy-wrap with the caption "It's true! I am going to be a mummy!"
She and her fiance, former NFL pro Eric Johnson, 33, who have been engaged since November 2010, had first child Maxwell on May 1.
Simpson, who became a spokeswoman for Weight Watchers in November 2011, shed about 50 to 60 pounds since giving birth by using the weight-loss company's PointsPlus program and four-times-weekly gym sessions with trainer Harley Pasternak, Us reported. "In over 20 years, I've never seen as consistent a weight loss," he told the magazine in October. "Jessica's right where she needs to be."
In a newly launched Weight Watchers TV commercial, the singer declares, "I didn't need to be perfect to get here, to lose over 50 pounds on Weight Watchers," adding: "I love food, I love life, and I had no idea I could be so in control and so free at the same time."