Delighted Rachel, 42, — mother to two-year-old Skyler — revealed the happy news this week on Twitter, writing, "Rodger, Skyler & I are beyond excited! :)"
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On her official website The Zoe Report, she added, "Before we head into the chaos of fashion season, we wanted to take a moment to OFFICIALLY confirm that we are expecting another child.
"We are beyond excited for Skyler to have a sibling and for us to fall in love all over again. We feel incredibly blessed."
Zoe gave birth to son Skyler in March 2011. After becoming a mum, the stylist said her approach to decorating and interior design changed.
"I used to think, 'Don't go in there with food!'. Then I learned that whatever is most precious in your house instantly becomes the kid's favourite thing," she told InStyle during a recent photo shoot of her Beverly Hills home.
"He spit up on my pristine couch at age six months. Now he uses it as a driveway for his cars."
It's been a week of celebrations for the star, who also marked her 42nd birthday on 1 September. Three days earlier, on 29 August, she celebrated her 22nd wedding anniversary with Rodger — a testament to the strength of their incredible bond.
Posting a picture of the couple on their special day, Zoe tweeted, "Happy anniversary to us!! 22 blissful years @rbermanus… xoRZ."
Zoe is considered the pioneer of the boho-chic look made famous by stars including Nicole Richie and Mischa Barton, and has dressed some of the biggest names in Hollywood, including Cameron Diaz, Eva Mendes, Anne Hathway, Jennifer Lawrence and Jennifer Garner.