"I found out at 10:15 and I had her at 11:06," Kropas told CNN affiliate WBZ
The Massachusetts woman said she woke up Tuesday morning with lower back pain so severe, she was rushed to a hospital in Weymouth, south of Boston.
"They told me that I had a full-term baby, ready to come now," she said.
Less than a hour later, Kropas gave birth to her 10 pound, 2 ounce daughter Ellie.
Kropas said the obvious pregnancy symptoms just weren't there.
Her mom Karen Kropas agreed, telling WBZ, "I couldn't believe it. I couldn't get my head around it, and I live with her. As a female, there were no signs."
Cryptic pregnancies, as they are called, aren't completely unheard of. Cable network TLC even created a show about them, "I Didn't Know I was Pregnant."
But exactly how common they are depends on who you ask.
According to one Serbian study
, an estimated one out of every 7,225 pregnancies is unknown to the mother until the moment of delivery.
Dr. Kim Dever is the chair of obstetrics and gynecology at South Shore Hospital, where Kropas gave birth. She said of the approximate 3,500 annual births at her hospital, "we probably see this a few times a year."
Dever said it often happens if the woman is overweight or has irregular periods. If the mother-to-be wasn't planning to get pregnant, she might confuse baby movement as gas, Dever said.
"I thought I had put on some Christmas-season weight, but I never thought I was pregnant. Never," Kropas told WBZ.
Without any time to prepare, Kropas said she expects the first few weeks with her daughter will be challenging, but worth it.
"I love her," she said.