Country music star Carrie, 31, broke the news on 1 May – Labor Day – playing off the aptly titled holiday.
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Carrie Underwood and her husband Mike Fisher are expecting their first baby
"In honour of 'Labor' Day, Ace & Penny would like to make an announcement. Their parents couldn't be happier," the singer captioned a picture of her two dogs, wearing 'I'm going to be a big brother' and 'I'm going to be a big sister' T-shirts.
Carrie and professional hockey player Mike, 34, celebrated their fourth wedding anniversary in July. The couple were married in Greensboro, Georgia in front of more than 250 guests, including Simon Cowell, Faith Hill and Garth Brooks.
Carrie Underwood shared the happy news by sharing this Instagram photo
The mum-to-be previously told Self magazine that she would love to have two children, adding, "Three is fine. Four? Somebody's getting something done, because we ain't having five!"
Carrie shot to fame as the winner of the fourth season of American Idol in 2005. She has since won six Grammy Awards and sold more than 15 million albums worldwide.