The woman had decided to terminate the pregnancy and underwent an abortion at Frölunda hospital in Gothenburg in the ninth week.
When she continued to feel off-colour and pile on the pounds she returned to the hospital and was told that the symptoms could by psychosomatic, according to a report in the Expressen daily.
But her insistence ensured that she was ultimately given an appointment with a gynaecologist at Östra hospital, almost three months after she had undergone an abortion.
The new examination determined that she was still pregnant, now in the 19th week.
"It came as a complete shock to me and all of my family," the woman told the newspaper.
Doctors were able to conclude that the the baby was healthy, but discovered a cyst that had gone undetected during her initial examinations.
The pregnant woman has now reported the matter to the National Board of Health & Welfare (Socialstyrelsen) on the advice of the gynaecologist.