Complaining of intense lower back pain, Katherine Kropas went to the hospital on Tuesday—and had the shock of a lifetime.

The 23-year-old woman from Weymouth, Massachusetts had no idea she was pregnant until doctors, trying to find out what was wrong with her, did an ultrasound.

"They took one look, and they rushed me off into labour," Kropas told ABC's Boston station WCVB. "I found out that I was having a baby at 10:15. She was born at 11:06 pm."

Kropas gave birth via Caesarean section to a ten-pound, two-ounce baby girl, whom she named Ellie.

"She's perfectly healthy," a dazed Richard Kropas, Katherine's father, said. "I've never seen a more healthy baby."

Kropas told NECN that she and her long-term boyfriend Dan Keefe were surprised when they learned she was pregnant as she said she was on birth control and had a "pretty regular" menstruation cycle.

According to Kropas, she also experienced no morning sickness or other telltale signs that might have signalled to her she was with child. She thought she had only put on weight over the holidays, and the swollen ankles she had days before giving birth she attributed to the 50 hours she spent on her feet at work as a catering supervisor in Quincy.

Cases like Kropas, though uncommon, are certainly not unheard of.

According to a 2002 Berlin study that looked into 29,462 births, one in 475 women were not aware of their pregnancy until they were about 20 weeks along. Researchers also found out that one in 2,455 did not realise their condition until they were well into labour.

"It tends to happen in women who are overweight or obese to start, may have irregular periods and are less aware of their bodies," Dr Jennifer Ashton, a practicing OB/GYN, told ABC News.

Though life has certainly thrown her a surprise, the new mother seems unfazed and ready to take on motherhood.

Surrounded by family in South Shore Hospital's maternity ward, Kropas said, "It'll be fun. I'll have lots of help."