"It's not just that they hide the pregnancy from their parents, they often become in denial of the pregnancy," he said.
"If you have a combination of any or all of those things, a pregnancy can go undetected, or the woman can be in denial of it if the implications to their life are so great," said O'Brien, a spokesman for the Royal College of Obstetricians and Gynecologist.
A study published in 2011 by Glasgow's Victoria Infirmary said that denial of pregnancy was more common than expected, suggesting it occurred in around 1 in 2,500 births.
In a 2002 German survey of Berlin obstetric hospitals, researchers found that 40 percent of women who didn't realize they were pregnant had seen doctors who also failed to spot the signs.
Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.