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Rare phenomenon - With twin births accounting for only 1.3% of births globally, it is fair to say that being a twin is not very common. Most of us only come across a handful of sets of twins during our lifetime, after all.

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More than expected - The number of pregnancies that start out as twins, however, is much higher than the number of twins who actually survive until birth.

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Number crunch -

According to one study, as many as 12% of all naturally conceived pregnancies might start out as twin pregnancies.

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The vanishing twin -

The same study suggests that in one out of eight of those pregnancies, one twin vanishes, resulting in a single birth.

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Correlation with left-handedness -

It has been suggested that for people who are left-handed, the probability that they started out as twins is even higher, because twins are more likely to be left-handed.

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Statistics -

While only 9% of the general population is left-handed, 15% of identical twins and 12% of non-identical twins are left-handers.

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Why twins vanish - We do not understand a huge amount about twins, in particular those that are identical. There are a few hypotheses, however, about why one twin might vanish.

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The hypothesis -

It may well be that something is wrong with one of the fetuses, or that the mother’s uterus simply does not have room to nurture both.

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Expert opinion -

According to psychology professor Nancy L. Segal, who specializes in twin studies, “The womb was designed for one baby, not two…”

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Continued -

“It could simply be a correction to ensure the health of the twin who survives.”

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Parasitic twins - In some cases, there is physical evidence of the twin that vanished. With parasitic twins, for example, the surviving twin absorbs the vanishing one. Parts of the vanishing twin remain in the surviving one, and you might be able to see a structure that resembles a spine, or even some hair or a limb.

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Very rare phenomenon -

Parasitic twins are very rare, however. It is estimated to occur in one out of 500,000 live births, and there have only been 200 cases reported globally.

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Going unnoticed - In most cases when a pregnancy starts out as a twin pregnancy and ends in single birth, it goes completely under the radar.

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Twin dreams -

And yet we often hear stories of people who are convinced they have memories of sharing the womb with a twin, or who have dreams in which they have a twin.

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Questionable merit -

Whether there could be any merit to these suggestions is still up for debate, but scientists may be close to a reliable test that shows whether we had a vanished identical twin.

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Pure chance -

Researchers did not set out to develop this test, but rather chanced upon it while trying to determine why identical twins develop in the first place.

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The reason for identical twins -

Identical twins occur in very few species and they don’t make sense from an evolutionary perspective. Offspring with genetic diversity make for a better chance of survival.

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Tangential discovery -

The exact reason for the development of identical twins remains unknown, but we do now have the never-expected side outcome of a test that shows whether we used to be one.

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Accuracy rate -

The test only works for identical twins, and currently it only has an accuracy rate of 60-80%. That may well increase, but we do not know whether it will ever be 100% accurate.

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Nonetheless -

That said, this is an exciting development in the field of twin research and it may have a number of wide-reaching implications.

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Identity crisis -

On an individual level, it may provide some comfort to people struggling with their identity and the possibility that they may have lost a twin.

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Closure -

According to Segal, she receives regular emails from people who say they feel lost and are searching for a lost twin. This test may be able to provide some sort of closure for those individuals.

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Medical research -

Being able to test for a vanished twin in apparent non-twins may also transform entire areas of medical research and healthcare.

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Cerebral palsy -

For example, the test could have positive implications in the field of cerebral palsy research.

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Cerebral palsy and twins -

Identical twins are more likely to suffer from cerebral palsy, as well as other congenital anomalies, such as neural tube defects and arthrogryposis.

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Treatment -

It may well transpire that singletons who suffer from these conditions started out as twins, and knowing that could be useful when it comes to treatment.

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Making inroads -

According to twin research expert Jenny van Dongen, "If we can determine that a significant number of patients with cerebral palsy used to have a monozygotic twin, then we may find better treatment targets."

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Moving forwards -

We may be a way off from having a test with a reasonable accuracy rate, but it seems fair to say that this is an exciting development in the area of twin research.

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All in all -

If it becomes more accurate and widely accessible the vanished twin test may provide answers to some important medical and psychological questions.

Sources: (BBC)

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