Underwood and Fisher are already parents to son Isaiah, 3.
She said: 'You might be wondering or asking, "Carrie, why is your tour starting in May?"'
"Mike and Isaiah and I are absolutely over the moon and excited to be adding another little fish to our pond", Underwood said.
"Ooh baby do we have a bundle of info to share with you!" The news came along with the announcement of Carrie's upcoming Cry Pretty tour, which won't kick off until 2019, even though her album will drop in September. Tickets go on sale August 17.
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The singer did not reveal how far along in her pregnancy she is or the sex of the child.
It's been quite a year for Underwood.
She also opened up about having "a few more downs than ups last year" after a scary fall left her with a broken wrist and 50 stitches in her face. After staying away from the spotlight for a few months while she healed, she released her single "Cry Pretty". Per USA Today, "those two sentences unleashed a commotion of emotions", including among some who viewed the remark as misguided, per US Weekly, or an insult to people with fertility issues who can't afford treatment like, as they pointed out, she could.