The strange case of semi-identical twins who survived despite "incompatibility with life"

It is such a strange event that it is only the second documented in the history of neonatology.

Doctors in the city of Brisbane, Australia, helped a woman give birth to a pair of "semi-identical" twins.

It's a boy and a girl, who are currently 4 years old, who are identical on the mother's side.

But they share only part of their father's DNA, which places them genetically between fraternal twins and identical twins.

Experts say the phenomenon is extremely rare because such embryos often do not survive.

Professor Nicholas Fisk, who led the team that took care of the mother and twins at the Royal Brisbane Hospital in 2014, said the discovery had been made by means of the following: a routine pregnancy test.

It was the first time that semi-identical twins were identified during pregnancy.

The first mother was then 28 years old and had naturally given birth, announced the doctors.

"The mother's ultrasound at six weeks showed only one placenta and the placement of amniotic sacs indicating that she was expecting identical twins," said Professor Fisk.

"However, an ultrasound at 14 weeks showed that the twins were a boy and a girl, which is not possible with identical twins."

The case was published in the trade journal The New England Journal of Medicine. Until now, the identity of the children has not been revealed.

How did it go?

Cases of identical twins, or monozygotic twins, occur when a single egg, fertilized by a single sperm, divides and forms two babies.

These twins will be of the same bad and will share the same genes and physical characteristics.

Non identical or dizygotic twins occur when two separate eggs are fertilized, each with a different sperm, and then develop in the uterus at the same time.

These twins can be of the same or different gender and are no more alike than any other sibling, even though they were born together.

In this case of twins semidénticos or sesquizigóticos, the researchers think that the egg was fertilized simultaneously by two spermatozoa before dividing.

If an egg is fertilized by two spermatozoa, we obtain three sets of chromosomes instead of the two standards, one of the mother and two of the father.

And, according to the experts, three sets of chromosomes are "generally incompatible with life and embryos do not usually survive".

Favorable factors

Non-identical twins are more common in some families.

Older mothers are also more likely to have non-identical twins because they are more likely to release more than one egg during ovulation.

The treatment of infertility can increase the possibility of twin production because several embryos can be placed in the uterus.

"Special case"

The first documented case of semi-identical twins was discovered in the United States in 2007.

Professor Fisk said that an badysis of databases of twins from around the world showed how rare sesquizigotic twins were.

His colleagues and himself examined the genetic data of 968 fraternal twins, as well as a series of major global studies, but found no other cases of sesquizygotic twins.

"We know that there is an exceptional case of semi-identical twins," concluded the expert.

"Although doctors can account for it in apparently identical twins, its rarity means that there is no basis for routine genetic testing."