A 30-year-old woman waddles into a family clinic with a large belly and tender breasts. She says she can feel her baby moving inside of her. A doctor performs a pelvic exam and discovers that not only is there no baby, there's no uterus. Her medical records show she'd had a hysterectomy two years earlier.

This case presented itself to Paul Paulman, a professor and family practitioner at the University of Nebraska. It was his first encounter with a rare condition called pseudocyesis, or false pregnancy. "I showed the woman a scan of her abdomen and explained the facts," Paulman says, "and then I never saw her again. I don't know if she ever accepted the truth."

In pseudocyesis, the mind tricks the body, and vice versa. Doctors think it develops when a woman obsesses over pregnancy out of desire or fear. (Queen "Bloody" Mary I of England famously suffered false pregnancy under pressure to continue the royal line.) A woman may stop menstruating, or her stomach may become distended due to stress or constipation. But her brain interprets the signs as pregnancy, which triggers the pituitary gland to secrete hormones like prolactin to prepare the body to carry a child. She gains more weight around the midsection, and her breasts swell and might even lactate. Many false pregnancies end when the woman goes into labor and delivers nothing.

Pseudocyesis occurs in only 1 to 6 of every 22,000 pregnancies, and it can also happen to children, the elderly, and men. "I think the men are a little more emotionally ill," Paulman says. Doctors confront the patient with medical evidence and offer counseling. If that doesn't work, the patient could have an underlying psychotic illness.

Pseudocyesis has a sibling syndrome: "couvade," or sympathetic pregnancy, where men experience many of the symptoms of their wives' or daughters' pregnancies—weight gain, nausea, headache, irritability, backaches, abdominal pain. A study of 81 expectant fathers found that almost half of them gained weight in the third trimester. Sympathy abdominal pains during birth are even more common, Paulman says. "I guess we all want to be in touch with our feminine side."

• The Patient

  A 36-year-old mother of four with a history of depression and hypomania.

• The Delivery

  Upon hearing that her 19-year-old son's girlfriend was going into labor, she began to experience excruciating pain in her abdomen. She felt as though she were having contractions, and after an hour, she gave "a final push." Afterward, she was exhausted, relieved, and overjoyed.

• The Cause

  The patient had been estranged from her son because she disapproved of his girlfriend and believed they were too young to have children. The phantom delivery may have been her way to include herself in the birth of her first grandchild.

• The Resolution

  After "labor," the grandmother felt ready to embrace her son, his girlfriend, and their baby.