Nicole Richie with daughter Harlow on September 16, 2009. Credit:

One week after welcoming son Sparrow, Nicole Richie is back to her svelte self.

Wearing a flowy maxi dress, the mother of two, 27, debuted her post-baby body Wednesday while running errands in Los Angeles with daughter Harlow, 20 months.

"Nicole was in a really good mood," says the X17 photographer who spotted her. "She looked absolutely amazing -- anyone who didn't know would never have guessed she had given birth just a week ago.  She even looked well-rested!"

See more pics of Nicole Richie's post-baby bod on

Sparrow James Midnight Madden - who weighed 7 lbs., 14 oz. - was born on Sept. 9

While pregnant, Richie admitted she would send beau Joel Madden out at "2 or 3 a.m. to pick up In-N-Out Burger."

Still no word on how she lost the weight, but she seems to be changing her diet. After giving birth, she Twittered: "Next stop: Sushi!"

Richie also lost the baby weight fast after having daughter Harlow in 2008.

At the time, Richie said she was breastfeeding and, as a result, had to watch what she ate. She said she mostly stuck to "bland" foods, like soup and fish.

The reality star -- who designs a maternity line for A Pea in the Pod -- also said she's "made adjustments" to her style after giving birth.

"Not all my clothes fit the same. I don't even think it's a weight thing; I think your body just changes after you have a baby," she told Harper's Bazaar in 2008. "I don't care either way."