Claudia Fernanda, 27, gave birth to a girl weighing 3.3 kilograms, with a height of 48 centimeters.
"I did not know I was pregnant," she told doctors in a maternity department, where she came on foot with her baby, the umbilical cord still attached.
Claudia Fernanda, who is unemployed and already has three children aged eight, six, and four, said she had visited a doctor a week before the delivery, but the examination had not revealed any signs of pregnancy.
Doctors say such occurrences are not rare, especially with overweight mothers.
Professor George A. Macones, head of the Department of Obstetrics and Gynecology at Washington University in St. Louis, a specialist in pregnancy complications, said he had witnessed over a dozen such cases in his 20-year career.
Last year, a woman in Washington was taken to hospital complaining of acute stomach pains, and was released some time later with a baby boy. She thought she had put on 14 kilograms after giving up smoking and eating too much.