Unique and Unusual Types of Twins

Most parents are familiar with the two types of twins - fraternal (dizygotic) and identical (monozygotic). Outside of those main classifications, there are some other rare and interesting types of twinning. These unique and unusual types of twins may not be commonly encountered, but they are interesting to learn about. 

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Ana and Tati Dogaru
Anastasia and Tatiana Dogaru. Photo courtesy of Rainbow Babies & Children's Hospital.

Conjoined twins are monozygotic multiples that do not fully separate from each other due to the incomplete division of the fertilized ovum. The individuals will be connected at certain points of the body and may share tissue, organs, or limbs.​

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Normally when an egg is fertilized, a woman's cycle is interrupted and ovulation ceases. Rarely, however, an egg can be released while a woman is already pregnant. This results in twins that are conceived at different times, which is known as superfetation.

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Fraternal (dizygotic) twins are the result of hyperovulation, the release of multiple eggs in a single cycle. Superfecundation describes a situation where the eggs are fertilized by sperm from separate incidences of sexual intercourse.

In a case where a woman has sex with different partners, the twins could have different fathers. The appropriate term for this is heteropaternal superfecundation.

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Dizygotic (fraternal) twins result when two eggs are fertilized. Monozygotic (identical) twins come from a single fertilized egg that splits. But what if the egg splits and then each half meets a sperm?

That's the proposed theory for polar body or "half-identical" twins, twins who are very much alike but aren't a 100 percent DNA match. Although it seems to be a reasonable theory, there is no definitive test to confirm polar body twinning.

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Identical (monozygotic) twins are always the same gender. This is because they form from a single zygote that contains either male (XY) or female (XX) sex chromosome.

However, there have been a few reported cases of a genetic mutation in male twins where one twin loses an Y chromosome and develops as a female. The female twin would be afflicted with Turner's Syndrome, characterized by short stature and lack of ovarian development.

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Mirror-image twins are monozygotic -- twins that form from a single fertilized egg. When the split occurs late -- more than a week after conception -- the twins can develop reverse asymmetric features.

This term is not really a type of twin, just a way to describe their physical features. For example, they may be right and left-handed, have birthmarks on opposite sides of their body, or have hair whorls that swirl in opposite directions. In theory, if the twins faced each other, they would appear to be exact reflections of each other.

About 25 percent of identical twins are mirror-image twins.

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Parasitic twins refer to a type of conjoined twins that develop asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin. One well-publicized example is that of Manar Maged who gained notoriety after being featured on Oprah.

A variation of parasitic twinning is ​"fetus in fetu," where an abnormally formed mass of cells grows inside the body of its monozygotic twin. It survives during pregnancy, and even occasionally after birth, by tapping directly into the blood supply of the host twin. This report from 2006 describes an Indian man whose fetus in fetu was discovered as an adult.

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Semi-identical twins is a type of twinning identified in a pair of three-year-old twins in 2007. Described as identical on the mother's side but sharing only half their father's genes, the rare twins developed when two sperm fertilized a single egg, which then split. One twin is a hermaphrodite being raised as a female, with both testicular and ovarian structures, while the other is anatomically male.

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The most common explanation is a labor and delivery that begins before midnight on one day and ends after the clock changes to the next day. If that day happens at the end of the month, or even on New Year's Eve/Day, the two babies can have birthdays in different months and even different years.

Also, sometimes a pregnancy is prolonged to provide each baby with an optimal chance for survival. If preterm labor forces the delivery of one baby, doctors can successfully control the labor and delay delivery to give the other baby more time in the womb.

Twins and higher multiples have been born days and even weeks apart.

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Twins of Different Races

Heteropaternal superfecundation can explain cases of fraternal (dizygotic) twins with differing racial characteristics. In one case, the differentiation was due to a lab mix-up during an in-vitro procedure.

However, in 2005 in the United Kingdom, two bi-racial parents conceived fraternal (dizygotic) twin girls, Kian and Remee Hodgson. Described as a "one in a million" occurrence, experts explain that the girls inherited different genetic characteristics from their mixed race parents. One is fair-haired and light-skinned while the other has dark hair, eyes, and skin.

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