The couple shared a beautiful photo with their son Jackson sitting in the foreground, with a shirt reading "Big Brother." Another photo in the gallery shows the couple holding an ultrasound.
"Zachary and I are so excited to announce that Jackson is going to be a big brother! We are expecting a sweet baby girl this November," Tori wrote in the caption. "Thank you so much for always supporting our family and loving us!"
Tori also tagged photographer Monique Serra, who has taken dozens of the adorable photos Tori has shared on her Instagram page. Serra shared a behind-the-scenes anecdote on the making of the photo shoot in her own Instagram post.
"What a fun three months this has been, getting to keep this amazing secret, pray for baby's growth and health, and getting to spend a night at the farm, watching Jackson stomp around with so much joy and pride wearing his 'big brother' shirt as we took announcement photos," Serra wrote.
The couple also told Us Weekly the baby is a girl and due in November.
"We are so excited to announce that Jackson is going to be a big brother!" the couple told the magazine. "We can not wait to meet our sweet baby girl!"
Zach previously told Us Weekly in April the couple was hoping to keep their family growing.
"We have plans," the reality TV star said at the time. "We want a family. I would love four or five kids. I want a little pack. Right now, we're just kind of [like], if it happens, it happens."
"It's just on God's timing and just whenever. We're ready for it," Tori added.
Zach's father Matt Roloff was among the hundreds of Instagram users to congratulate the couple on their new addition.
"Grandpa is hitting the lotto," Matt joked.
Audrey Roloff, the wife of Zach's brother Jeremy, also congratulated the two. Jeremy and Audrey are parents to 1-year-old daughter Ember.
"Wahoooooo CANT WAIT FOR THE GIRLSIES TO HANGGGGG I'm so happy for you Tori," she wrote.
While the couple waits for their new baby, they are also enjoying watching 2-year-old Jackson grow. Zach told Us Weekly his son is "running a lot more" and is beginning to understand more.
"He's talking more, he's mimicking more [and] he can understand more," the TLC star said. "He's just doing everything a little bit better lately."0comments
Photo credit: Instagram/Tori Roloff