According to a foreign news agency, Yara Bernardo Jaco, 20, said that she went to doctors in Cristalina, 80 miles (130 km) from the capital Brasilia in central Brazil, complaining of pain.
However, a doctor noticed nothing unusual.
Jaco went home and just hours later felt a need to go to the toilet — and the baby boy was born.
It has been reported that the doctors have said that Jaco was eight months pregnant. The infant, who weighed five pounds (2.3 kilograms), is in good health.