Exceptionally rare 'semi identical' twin babies have bewildered doctors

This is an extremely rare case. The second in the world, and the first to have been detected during pregnancy: these ‘identical twins’ have turned out to be of different sexes. A little girl and a little boy, who at the beginning of their life as embryos were believed to be identical.

‘Their mother's ultrasound at six weeks [of pregnancy] showed a single placenta, and a positioning of the amniotic sacs that indicated that she was expecting identical twins. But an ultrasound performed at 14 weeks showed that the twins were of the genders male and female, which is not possible for identical twins,’ saysProfessor Fisk, director of the service where the mother of the twins was monitored during her pregnancy, at Royal Brisbane and Women's Hospital in Australia.

‘Two small seeds’ from dad

To explain this unique case, the doctor put forward a hypothesis. Professor Fisk explained:

It is likely that the egg from the mother was fertilised simultaneously by two of the father's spermatozoa before division.

For these ‘semi-identical twins’ a single egg appears to have been fertilised by two sperm cells. A tripartite meeting which is unusual, and which is not inconsequential on the genetic material of future babies.

Viability usually compromised

‘If an egg is fertilised by two spermatozoa, it results in three sets of chromosomes, one from the mother and two from the father. [Possessing] three sets of chromosomes are normally incompatible with life, and the embryos do not usually survive’ says another doctor in the department, Dr. Gabbett.

This biological prodigy makes these Australian twins the second in history to have lived such a genetic adventure; their only predecessors were born in 2007 in the United States. Dr Fisk concluded:

We know that this is an exceptional case of semi-monozygotic twins, and although doctors must keep this in mind in the apparent cases of identical twins, the rarity [of the phenomenon] does not justify the achievement of a systematic genetic test

This is an extremely rare case, and it would seem that it could remain so for a long time yet.

Take a look at the video above for more on this extraordinary story.