She captioned the adorable picture of her children in a field with a simple: "SURPRISE..."
Jessica's fans expressed their excitement over her baby news, with one of her followers writing on her Instagram: "Congratulations girl!"
"This little baby girl will make us a family of five", she wrote, featuring her son and daughter surrounded by pink balloons. They welcomed their first child, Maxwell, in 2012 and their second, Ace, in 2013.More news: 'Mrs. Maisel' takes home seven Emmy awards
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"We always practice", Simpson added of having another baby.
Fans were quick to comment on all three of her posts and congratulated both the star and her husband of four years on the news. Hope you're feeling great'. It looks like you have two very excited older siblings!'
She told Us Weekly earlier this year that how being a mum is the "best opportunity and challenge" she has been blessed with. "I have two attractive children and I'm not having a third".
"Right now my kids are in this incredible stage where I'm really listening to them without judgment and they're teaching me so much".