At first, everything seemed normal for one 28-year-old mother who was pregnant for the first time in Brisbane, Australia. An ultrasound at six weeks revealed a small but common surprise: a single placenta and two amniotic sacs. She was pregnant with identical twins.

Eight weeks later, however, a follow-up ultrasound exposed a much more striking origin story. The identical twins were a boy and a girl – an impossible discovery for twins with a shared placenta.

"We could see very clearly that they appeared to be identical twins from very early on in the pregnancy, so it came as quite a shock to later discover that one was a boy and the other a girl," Dr Michael Gabbett, clinical geneticist at Queensland University of Technology (QUT), told IFLScience.

To investigate further, the doctors performed genetic testing of the amniotic fluid in each sac. The tests revealed the twins are identical in some respects but not in others. The boy and girl share 100 percent of their mother’s DNA, but only about 50 percent of their father's, resulting in 78 percent shared genetic information.

The unique duo are only the second semi-identical pair known. This means they are not monozygotic (identical) or dizygotic (fraternal) but “sesquizygotic" (semi-identical). The discovery is described in The New England Journal of Medicine.

So how did this genetic mashup happen?

The semi-identical twins are a sesquizygotic cross between identical and fraternal pairs. Identical twins result from a single sperm fertilizing a lone egg, with the cells subsequently dividing in two. These individuals share 100 percent of their DNA.

Fraternal twins develop when two eggs are fertilized by different sperm, each becoming a distinct embryo. These twins have a separate placenta but share a womb, ending up with about 50 percent similar DNA, just like non-twin siblings.

The sesquizygotic twins are an “exceptional intermediate”. They are genetically identical with respect to their mother but differ by 50 percent in the DNA they inherited from their father. The medical team dubbed this “a third form of twinning.”

The Brisbane twins are likely the result of one egg being fertilized by two sperm cells, resulting in three chromosomes. These three chromosomes were equally divided into two bunches of cells that then became two embryos. Years later, the fetuses are now 4-year-old toddlers. Phew, what a process!

The twins were delivered by cesarean section and had normal Apgar scores, a quick assessment of newborns in the minutes after birth. Due to medical complications, the young girl had her right limb amputated at 4 weeks of age. At 3 years, she was also diagnosed with gonadal dysgenesis, a congenital developmental disorder that meant her ovaries had to be removed.

“Otherwise, both twins [are] developmentally normal,” wrote the authors.

Is this a rare event?

Yes, it seems so. This is only the second-known time semi-identical twins have been recorded. The first were reported in 2007 – they too are identical on their mother’s side but fraternal on their father’s side. However, these kids were not identified as semi-identical until years later, when one was noted to have ambiguous genitalia, with both ovarian and testicular tissue. The other was anatomically male. They too appear to be doing well and growing normally.

"The standard teaching is that when an egg cell is fertilized by two sperm cells, the baby receives three sets of chromosomes," said Dr Gabbett. "We do see this occasionally in pregnancy and rarely in newborns however these babies usually die early on in the pregnancy. What this research demonstrates is that it is possible for nature to correct this event so that the pregnancy can survive and result in relatively healthy, developmentally normal children."

Does this mean our current classification of twins is oversimplified?

"I think our current classification of pregnancy serves us well and does not need to be changed," said Gabbett. "However, biology will always throw us unexpected surprises and we need to be aware that possibilities beyond our regular classification systems will always exist."