This is only the second known case in the world, and the first time it has been identified during pregnancy using genetic testing.

In the case of fraternal (or dizygotic) twins, two separate eggs are fertilized by two separate sperm, so the resulting embryos share half of their DNA. Identical (or monozygotic) twins form when a single egg, fertilized by a single sperm, splits in two, creating embryos that share 100 percent of their DNA and are always the same sex.

In extremely rare cases there is a third intermediate form, known as semi-identical (or sesquizygotic) twins. This occurs when one egg is fertilized by two sperm at the same time, splitting into two the same way as identical twins but each carrying different genetic information from the father.

As a result, semi-identical twins share three quarters of their DNA – 100 percent from the mother's side, since they both hail from the same egg, but a different amount matching on the father's side, since they came from two different sperm.