"Of course you wonder if it's you, what am I doing wrong, or what have I done wrong. I remember having conversations with Mike trying to make sense of it all," the star told the magazine.
Underwood, a devout Christian, struggled with her losses but ultimately put her future in God's hands.
"I've always wanted to be a good daughter to my parents but also to God and not complain, because we are beyond blessed. I get to do what I love, I have an incredible family," the singer said. "I have all of these amazing people around me, and I don't want to complain, ever."
"But the miscarriages made me get real with God and say, 'Okay, I'm kind of giving up a little bit. If this isn't meant to happen, then I need to accept that and know that someday I'll understand why,'" she recounted.
Underwood got pregnant soon after and gave birth to Jacob in January. She celebrated Father's Day with Fisher, Jacob and 4-year-old son Isaiah on Sunday.
"To all the frog catching, bedtime story reading, wrestling, diaper changing, outside playing, adventure seeking, tickling, cuddling, loving dads out there, Happy Father's Day! @mfisher1212 your boys sure do love you and are so lucky to have you!" the star wrote on Instagram.