Those who suffer from the disorder present a constellation of symptoms that mystify even seasoned practitioners. Not only do they fervently believe they are pregnant, but they also have bona fide symptoms to back up their claims, like cessation of menstruation, abdominal enlargement, nausea and vomiting, breast enlargement and food cravings.

A few patients with pseudocyesis even test positive on pregnancy tests, said Dr. Paul Paulman, a family practitioner at the University of Nebraska Medical Center.

“Every sign and symptom of pregnancy has been recorded in these patients except for three: You don’t hear heart tones from the fetus, you don’t see the fetus on ultrasound, and you don’t get a delivery,” Dr. Paulman said.

Though the disorder is unusual, cases of false pregnancy have been reported in human societies since ancient times, providing evidence that the phenomenon is not bound by time or culture. In 300 B.C., Hippocrates described 12 women who “believed they were pregnant,” and Mary Tudor, the English queen, is widely believed to have suffered from pseudocyesis. (Some commentators say the violent acts that gave her the nickname Bloody Mary were reactions to finding out she was not carrying an heir after all.)

For hundreds of years, pseudocyesis has largely been the domain of psychiatrists, spawning many psychological theories about the origins of the condition. According to Sigmund Freud’s memoirs, his most famous patient, “Anna O,” believed she was pregnant with the child of her previous psychoanalyst, Josef Breuer. Freud attributed this development to what he called transference, or the strong attachment patients form with their psychoanalysts — a concept that would later form the cornerstone of Freudian theory.

Other psychiatrists have suggested that pseudocyesis occurs in patients who desperately want to become pregnant — or who have a strong desire to be involved in a family member’s pregnancy experience. In a recent issue of the journal Psychosomatics, Dr. Biju Basil, a psychiatrist at Drexel University, reported a case of a woman who went through false delivery at the same time her son’s girlfriend was giving birth.

“She started having labor pains. She had been pregnant before, and she said they felt exactly the same,” Dr. Basil said. “She even gave a ‘final push’ the way she had when she delivered her own children.”