Have you heard of these rare and unique twin types?

When it comes to twins, you probably think of either Dizygotic (fraternal) twins that result when two eggs are fertilized or Monozygotic (identical) twins that come from a single fertilized egg that splits. But did you know there are other rare and unique types of twins? Check out our run-down of these unique twin types that are pretty unusual and rare in our twin world.

Polar Body Twins or “Half Identical”

What happens when the egg splits and then each half meets a sperm? That’s what scientists and researchers propose happens when a polar body or “half-identical” twins form. These twins are very much alike but aren’t a 100% DNA match — sharing about 75% of their genetic DNA. (Less than identical twins but more than fraternal twins.) To date, polar body twinning remains a theory. Believe it or not, there are no definitive tests to confirm whether you are carrying polar body twins even though there have been confirmed cases.

Semi-Identical Twins

This unique twin type identifies as identical on the mother’s side but shares only half their father’s genes! Semi-identical twins develop when two sperm fertilize a single egg, forming a triploid, which then splits. In 2007, the Journal of Human Genetics described a case where one hermaphrodite twin (someone born with a discrepancy between their internal genitalia and external genitalia) was raised as a female, with both testicular and ovarian structures, while the other was anatomically male.

Boy/Girl Monozygotic (Identical) Twins

Did you know identical twins are almost always the same gender because they form from a single zygote containing male (XY) or female (XX) sex chromosomes? However, there are a few reported cases of a genetic mutation in some male twins where one twin loses a Y chromosome and develops as a female. The female twin would then be afflicted with Turner’s Syndrome, a genetic disorder in girls caused by a missing or defective X (female) chromosome. It occurs in 1 of 2,000-2,500 live female births. There are many different features associated with the syndrome, and not all girls have all symptoms. Still, typically they are short in stature and lack ovarian development, have hearing disorders, abnormal fingernails, toenails, and drooping eyelids (Ptosis). The only other time gender differences would occur in identical twins would be if one twin experienced a gender transition process.

Mirror Image Twins

Mirror image twins make up around 25% of identical twins. This unique twin type occurs when the fertilized egg splits late – more than a week after conception. This type of twins can develop reverse asymmetric physical features, such as birthmarks or swirling hair whorls on opposite sides of their bodies. So, when these types of twins face each other, they appear to be exact reflections (like looking in a mirror).

Superfetation: Twins Conceived Separately

Typically, when a woman’s egg is fertilized, her cycle is interrupted, and her ovulation ceases. Rarely, an egg can be released while she is already pregnant, resulting in twins that are conceived at different times! This happens when eggs from two separate menstrual cycles are released as opposed to normal fraternal (Dizygotic) twins, where multiple ova are expelled in a single cycle. This is a rare occurrence in humans but is common in animals and usually results in twins or higher-order multiple pregnancies where the fetuses show noticeable differences in gestational development. Sounds crazy, but this Arkansas mom was pregnant with two babies conceived about two and a half weeks apart.

Heteropaternal Superfecundation: Twins with Different Fathers

Yes, you read that right. Twins with different fathers are possible. Sure, the definition for twins clearly states, “Twins are two babies who are simultaneously born from one mother.” But what about the dad? We know that when a woman releases multiple eggs in a single cycle, known as hyper-ovulation, it results in fraternal twins. However, superfecundation happens when the eggs are fertilized by sperm from separate incidences of sexual intercourse. In a nutshell: A woman with multiple sexual partners could have twins with different fathers (heteropaternal superfecundation). Genetic testing and advances in technology have made it evident that twins with two fathers only apply to fraternal twins, not identical twins. This situation can also occur when twins are the result of fertility treatments. This happened in 1995 with a pair of IVF Dutch twins. There was a mix-up in the lab when a piece of lab equipment had been accidentally used twice, causing another man’s sperm to be mixed in with the father’s and fertilized with the mother’s sperm. The result: two babies of different races.

Twins of Different Races: So rare that the odds against this are a million to one!

As mentioned above, heteropaternal superfecundation explains cases of fraternal twins with differing racial characteristics. The case above was due to a lab mix-up during an IVF procedure. However, in the United Kingdom back in 2005 and described as a “one in a million” occurrence, fraternal twin girls were conceived from two bi-racial parents. Each inherited different genetic characteristics from their mixed-race parents. (Essentially, one was white, and the other was black.) For this rare occurrence to take place, there must be a combination of circumstances starting at the exact moment of conception. This phenomenon can occur when both parents are of mixed race; each is conceived from a separate egg fertilized by a separate sperm (fraternal); and each sperm and egg must carry the genes for particular skin color (i.e., black/black or white/white).

Conjoined Twins

In short, conjoined twins are identical twins that do not fully separate from each other due to the incomplete division of the fertilized egg. The individuals will be connected to specific body parts and may share tissue, organs, or limbs. As the Children’s Hospital of Philadelphia states, “Conjoined twins occur once in every 50,000 to 60,000 births.”

Parasitic Twins

Parasitic twins are conjoined twins that develop asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin. In Cairo, Egypt, back in 2005, a case of parasitic twins gained worldwide notoriety and even featured on The Oprah Show. Manar Maged was born with a second head attached to the skull of her own. While the head could blink and smile, it was not capable of independent life. The weight of the appendage would prevent Manar from crawling or sitting upright, prompting surgeons to remove it when she was ten months old. On February 19, 2005, doctors performed a thirteen-hour surgery to remove the attached head. The girl was released from intensive care in March 2005. Unfortunately, she passed away from a brain infection shortly before her second birthday on March 25, 2006. A variation of parasitic twinning is called “the fetus in fetu.” This occurs when an abnormally formed mass of cells grows inside the body of its identical twin. These cells survive during pregnancy and even occasionally after birth by tapping directly into the host twin’s blood supply.

Curious to find out more? Check out this report on a man who discovered a fetus in fetu as an adult.

The bottom line

Although most of us are familiar with fraternal and identical twins, many aren’t aware of the other unique twin types out there. After reading about these rare and unique twin types, you may have discovered a whole range of unique twin combinations that you may not have known were possible. But one fact remains true, regardless of what type of twin you are or have, the birth of twins is a miracle indeed.