A 19-year-old woman in Brazil has been left 'surprised' after she gave birth to twins by two separate fathers.

Medics in Brazil said this would be the twentieth known case of "heteroparental superfecundation". This rare case can happen in one of two ways.Firstly, it can happen if a woman has sex with two different men prior to her ovulation and the sperm are present in the fallopian tubes when two eggs are released.Secondly, it cam happen if a woman has sex with two different men after ovulating and the sperm are then able to fertilise two eggs.

The rare event can happen if sperm survives for a few days in the female reproductive tract, and a woman produces two eggs at once.But the chances of this happening are very unlikely as it would mean sperm having to survive for days from both men and a double egg release in the same cycle.The unnamed woman from southwest Goie's gave birth to twin boys.Doctor Telio Franco, who had been treating the woman, said her pregnancy had "been smooth"."The boys were born healthy and never had health problems. "It's extremely rare. It happens one in a million.

I never imagined that I would see a case like this in my life".The mother of the twin boys was understandably shocking and explained that she didn't know this could happen."I was surprised by the results. I didn't know this could happen. They are very similar," she added. "The woman decided to seek clarification on her twins" paternity as doubts began to arise when the children were 8 months old.

She took paternity tests for them both however the result showed positive for just one child.She then remembered that she had had sex with two men on the same day and the other man was the father of the other child.The mother now says one of the fathers acts as dad to both boys and has even made this official on record.According to her, the father takes care of both of them and gives them equal love and support.The rare medical phenomenon is mostly common in other animals, like dogs, cats and cows.