A Massachusetts woman who learned that she was pregnant at 10:15 a.m. on Wednesday gave birth to a healthy baby girl 51 minutes later, CBS News reported. Katie Kropas did not know she was bearing a child and had rushed to a hospital because of "crazy lower back pain."

Her baby girl, Ellie, was born at Boston's South Shore Hospital and weighed 10 pounds, 2 ounces. Kropas seems to have experienced what medical experts call a "cryptic" pregnancy, which South Shore's chair of obstetrics and gynecology said is rare but not unheard of.

"We do about 3,500 births a year, and we probably see this a few times a year," Dr. Kim Dever said. "Very often, especially in your first pregnancy, you're really not aware of what you're feeling, and we often have women joke and say they're feeling gas pains or they can attribute it to other natural conditions."

"Cryptic" pregnancies are particularly common in women who are overweight or have irregular periods, the physician explained, and in many cases, they may not feel the baby move because of how their placenta is positioned.

Some women simply "do not know they're pregnant because they have no symptoms -- no weight gain, no nausea and little to no abdominal swelling," science writer Jena Pincott explained in a Huffington Post blog. "If they have symptoms, they're so subtle as to be easily mistaken for something else."

"I thought I had put on some Christmas season weight, but I never thought I was pregnant. Never," Kropas said. While heartily surprised, she was happy to welcome the addition to her family, the Huffington Post noted. The 23-year-old settled on Ellie as the baby girl's name, an homage to her grandmother.

"They told me I had a full term baby, ready to come, now," she said. "I was like, 'This has got to be a joke,' but I had no idea what to think," she told local news outlet NECN.

Similarly stunned was Kropas's mother, Karen.

"I couldn't believe it," Karen Kropas said. "I couldn't get my head around it, and I live with her! As a female, there were no signs."