Sheehan's syndrome, which is pituitary necrosis after severe postpartum hemorrhage and hypovolemia, may cause hypopituitarism immediately or several years later, depending on the degree of tissue destruction. We report an unusual case, in which a 55-year-old woman with Sheehan's syndrome got worse symptoms after spontaneous labor. In 1998, she had severe postpartum hemorrhage and pituitary necrosis during the third delivery, thus it was diagnosed as Sheehan's syndrome by clinical manifestations, laboratory tests, and magnetic resonance imaging. She was treated by replacement therapy including hydrocortisone and levothyroxine sodium. However, she had the fourth spontaneous pregnancy in 2000 and got worse symptoms after delivery. We carefully concluded that pregnancy provided no evidence against the diagnosis of Sheehan's syndrome because pregnancy might improve hypopituitarism by stimulating the pituitary remnant to undergo hyperplasia and irritating the placenta to secrete hormone. However, pregnancy might aggravate the symptoms by inducing autoimmunity as well. All in all, early diagnosis and adequate medical treatment are important to provide a good prognosis of Sheehan's syndrome.