When most people think of twins they think they are 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 there are other types of rare and unique types of twins that you may not have heard about. Here is a run-down of some of the twin-types that are quite 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 polar body or “half-identical” twins are formed… twins who are very much alike but aren’t a 100% DNA match sharing about 75% of their genetic DNA which is less than identical twins but more than fraternal twins. To date, polar body twinning remains a theory and there are no definitive tests to confirm whether you are carrying polar body twins even though there have been cases confirmed.
A type of twinning identified as identical on the mother’s side but sharing only half their father’s genes, these rare twins develop when two sperm fertilize a single egg, forming a triploid, which then split. In 2007, the Journal of Human Genetics described cases where one twin was a hermaphrodite (someone who is born with a discrepancy between their internal genitalia and external genitalia) being raised as a female, with both testicular and ovarian structures, while the other is anatomically male. Semi-identical twinning is distinguished from Polar Body twinning because the egg was fertilized by two sperm before splitting.
Boy/Girl Monozygotic (Identical) Twins
Identical twins are always the same gender because they form from a single zygote that contains either male (XY) or female (XX) sex chromosome. In the past, there have been 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 but typically they are short in stature and lack ovarian development, have hearing disorders, abnormal fingernails, toenails and drooping eyelids (Ptosis). The only other time where there would be gender differences in identical twins would be if one twin experienced a gender transition process.
Mirror Image Twins
Mirror image twins makeup about 25% of identical twins and happen when the fertilized egg splits late – more than a week after conception. This type of twins can develop reverse asymmetric physical features like having birthmarks on opposite sides of their body, hair whorls that swirl in opposite directions and may be right and left handed. So when these type of twins face each other, they would appear to be exact reflections of each other like looking in a mirror.
Superfetation: Twins Conceived Separately
Typically, when a woman’s egg is fertilized, her cycle is interrupted and her ovulation will cease. However, rarely, an egg can be released while she is already pregnant, thus 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 quite a rare occurrence in humans but is quite common in animals and usually results in a twin or higher-order multiple pregnancies where the fetuses show an obvious difference in gestational development. There was a case back in 2009 when a woman became pregnant with two babies due to superfetation. Ultrasound revealed that this Arkansas mom was pregnant with two babies conceived about two and a half weeks apart.
Heteropaternal Superfecundation: Twins with Different Fathers
When you read the definition for twins it says… “Twins are two babies who are simultaneously born from one mother.” (Encyclopedia Britannica) You can see that it only refers to the mother but what about the fathers? We know that when a woman releases multiple eggs in a single cycle that is known as hyper-ovulation and result in fraternal twins. Superfecundation happens when the eggs are fertilized by sperm from separate incidences of sexual intercourse. Like in cases when a woman has sex with different partners, ultimately the twins could have different fathers and the term is called heteropaternal superfecundation. Genetic testing and advances in technology have made it more evident that twins can have two different fathers and only applies to fraternal twins, not identical twins. This situation can also occur when twins are the result of fertility treatments. In 1995, there was one case of Dutch twins that were a result of In Vitro Fertilization or IVF and there was a mix up in the lab when a piece of lab equipment had been accidentally used twice which caused another man’s sperm to be mixed in with the fathers and was fertilized with the mother’s sperm. When delivering their twins, they immediately knew something was not right when one baby was white and the other baby was very dark. You can read more about this case here which was covered by Dateline NBC.
Twins of Different Races: So rare… the odds against this are a million to one!
As mentioned above, Heteropaternal superfecundation can explain 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 and each inherited difference genetic characteristics from their mixed-race parents. So essentially one is white and the other is black. For this rare occurrence to take place there must be a combination of circumstances starting at the exact moment of conception. This can happen when both parents are 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 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 at certain points of the body, and may share tissue, organs or limbs. The occurrence is rare and is estimated to be about 1 in 200 sets of identical twins are born conjoined or about 1 in 85,000 births.
A type of conjoined twins that develops asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin. In Cairo, Egypt back in 2005, there was a case of Parasitic twins that gained worldwide notoriety and even featured on the Oprah Show. Manar Maged was born with a second head attached at the skull to 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. A thirteen-hour surgery was performed on February 19, 2005, to remove the attached head. The girl was released from intensive care in March 2005. She passed away from a brain infection shortly before her second birthday on March 25, 2006.
A variation of parasitic twinning is the fetus in fetu, where an abnormally formed mass of cells grows inside the body of its identical twin. It survives during pregnancy, and even occasionally after birth, by tapping directly into the blood supply of the host twin. This report describes an Indian man whose fetus in fetu was discovered as an adult.