At first glance, the woman rushing into the labor and delivery ward late at night appeared like the rest. She had a big belly and carried a bulging overnight bag.
But a nurse struggled to find a baby's heartbeat as she passed her stethoscope over the abdomen of the Pittsburgh brunette, who appeared to be middle-class and in her mid-30s. Then a doctor performed an ultrasound. Definitely no baby.
"Most people are convinced they're like 38 weeks along and they come in thinking they're in labor -- heavy breathing, the whole nine yards, with all their things," said Dr. Kimberly Gecsi, an obstetrician at University Hospitals in Cleveland, who over the past decade has seen "four or five" cases of a very rare false pregnancy disorder.
When asked whether she had gotten prenatal care, the woman admitted she hadn't, according to Gesci. Her husband looked at his wife in disbelief. Their whole house was decorated for the arrival of a new baby.
More often, cases of the opposite sort make headlines: a woman who doesn't know she's pregnant gives birth. Earlier this month, a 44-year-old Michigan woman arrived at a hospital thinking she had a hernia then delivered a 10-pound-baby later that day.
But doctors also report cases of women who believe they're in late-term pregnancy but aren't.
In classic instances of the vanishingly rare condition, known as pseudocyesis, these women even have pregnancy symptoms, everything from an elevated presence of pregnancy hormones to enlarged breasts.
"The only ones (not present are) heart tones of the baby, an actual picture of the baby, and delivery," said Dr. Paul Paulman, professor of family medicine at the University of Nebraska College of Medicine. "Everything else has been shown."
But the fact that a woman's beliefs alone could make her body act as if it's pregnant, according to Paulman, makes some sense because the pituitary gland, a pea-sized structure at the base of the brain, helps control menstrual cycles and milk secretion.
"The brain decides to be pregnant," he said. "The good news is as far as physical harm, unless you're having a c-section you probably won't get hurt," Paulman said.
Doctors sometimes do get fooled. Two years ago an emergency c-section was performed on a woman who wasn't pregnant after doctors tried to induce her for two days at a North Carolina hospital. Two physicians were disciplined.
There aren't any reliable statistics that could help explain how commonly women get pseudocyesis, according to Dr. Orit Avni-Barron, a psychiatrist and director of The Fish Center for Women's Health at Brigham and Women's Hospital. Typically the condition occurs in women ages 20 to 39 and it has been observed in women of all races and income levels in this country, said Avni-Barron, who published a report on pseudocyesis in 2010. Research that does exist is based only on case studies -- no randomized trials, Avni-Barron said.
"It's really impossible to track," he said. "The people really don't want to hang around and answer a bunch of questions after they find out they're not pregnant."
Gecsi said women often get embarrassed after finding out the truth. Typically they accept the fact and go home to "normal lives," she said.
"You're kind of relieved that it's not the alternative, that she was pregnant and the baby died," Gecsi said. "You feel bad. In her mind, it's a loss … it's sad."
Pseudocyesis isn't a recent phenomenon or even one limited to humans. The illness, classified as "somatoform disorder" in the Diagnostic and Statistical Manual of Mental Disorders, a reference manual published by the American Psychiatric Association, has been observed in other mammals such as dogs. Even medieval writings refer to the condition.
Many historians argue that the English queen Mary Tudor, known as "Bloody Mary," suffered from pseudocyesis and ordered repeated burnings at the stake in part out of stress from an inability to produce an heir. Today, shows including "Glee" and "Law and Order" have featured characters suffering from what appears to be pseudocyesis.
Pseudocyesis is most common in developing nations where large families are valued and a woman's identity is tightly wrapped up with being a mother, Avni-Barron said.
"It's almost a social disorder," she said. "It's fascinating."
The condition appears to be rarer today in the developed world as family sizes have shrunk and a woman's primary role is no longer only to raise children, she said.
"Having kids is not such an important factor (here) as in other cultures," Avni-Barron said. However, "immigrants can be more at risk," she said.
Risk factors include a strong desire to have a baby, low self-esteem, and a tendency to misinterpret things and come to conclusions easily, she said. If depression is present, it can affect neurotransmitters such as serotonin which interact with reproductive hormones to "cause a real change" in a woman's body, Avni-Barron said.
Doctors who encounter such women should clearly explain to them the facts of the situation, said Paulman, who once encountered a woman who had undergone a hysterectomy but still believed she was pregnant. He had to show her an ultrasound and tell her, "the equipment isn't there," he said.
But sometimes even showing an image of a vacant uterus isn't enough to shake the beliefs of a woman -- or her body's mechanisms.
One New York City area woman told her doctor that her baby wasn't visible in the ultrasound because it was hiding under her ribs, a friend of the woman, who asked to be quoted using only her middle name, told ABCNews.com.
The woman's insistence about her condition is alienating everyone around her, her friend said.
"It's just a big, big burden on my heart and my friends," Joy said. "We don't know what to do. This is a young person (and) it's going to destroy her life."
Her friend who she says suffered from the condition -- a former personal trainer in her mid-20s who is thin and always has been -- has a history of lying and exaggerating details of her life, Joy said. After repeated pregnancy tales in recent years her friends thought her latest pregnancy claim was just another ruse. Now, making matters worse, she really looks pregnant, Joy said.
"She sent a video to a friend recently to try to prove (her) pregnancy," Joy said. "There's a belly, there's no mistaking it. She looks like she's showing six months, it's disturbing. She picked up her shirt in the video and did a 360 of it."