In a rare case, a Portuguese woman, 19, has given birth to twins from two fathers after having sex with them on same day, in what is believed to be the 20th case of 'heteroparental superfecundation' ever recorded.

The mother, who preferred not to be identified, is from the town of Mineiros in the state of Goias in Portugal. When a DNA test was taken of the two children, the result came as positive for only one of her sons.

Representational image.Creative Commons.

The woman said: "I remembered that I had had sex with another man and called him to take the test, which was positive... I was surprised by the results. I didn't know this could happen and the babies are very similar."

Scientifically, a pregnancy that produces twins with two different fathers is 'heteroparental superfecundation'. However, the birth certificate has been registered in just one man as father for both children.

"He takes care of both of them, helps me a lot and gives them all the necessary support that they need," the woman told media.

Dr Tulio Jorge Franco, who has been studying the unusual pregnancy, said it is the 20th known case of heteroparental superfecundation in the world records.

Representational image.Creative Commons.

Here, two sperm have fertilized a single egg called 'double fertilization', a rarity and is thought to happen in about 1% of human conceptions. An embryo created this way is rare that so far 20 cases have been recorded in the world and these children are chimaeras of cells with X and Y chromosomes.

Rare Phenomenon

The doctor explained the Portuguese news outlet G1 that it happens when "two eggs from the same mother are fertilised by different men. The babies share the mother's genetic material, but they grow in different placentas".

He added: "The pregnancy was super smooth, without any complications. The boys were born healthy and have never had health problems... It's extremely rare. It happens once in a million. I never imagined that I would see such a case in my life."

The twins are now one year and four months old.