I broke down this very scientific article on “types of twins”. I added a little of my own take on it, but if you want to see the orginial it is :
http://multiples.about.com/od/funfacts/tp/uniquetwins.htm
1. Conjoined Twins
Are twins that are physically joined at different parts of their body. It could be tissue, organs or limbs.
2. Twins Conceived Separately: Superfetation
Usually when a woman becomes pregnant she doesn’t release any more eggs, but sometimes an egg can be released when she is already pregnant—then if she has intercourse during that … voila! Two separate pregnancies but within the same womb.
3. Twins with Different Fathers: Heteropaternal Superfecundation
Just think about Superfetation… but with different sperm owners. So you would get two twins (somewhat varying in age and which different “dads”).
4. ”Half Identical” twins: Polar Body Twins
The concept is somewhat not really confirmed as true or not. The idea is: Fraternal twins result when two eggs are fertilized. 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% DNA match. Although it seems to be a reasonable theory, there is no definitive test to confirm polar body twinning. (I don’t get it myself).
5. Boy/Girl Identical Twins
Identical (monozygotic) twins are always same gender 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. Of course, another explanation for gender differences in identical twins is an identical twin who undergoes sexual reassignment surgery. (A twin who decides to transition).
6. Mirror Image Twins
Mirror image twins identical twins that have their “split” a little later than most twins… like a week or so. As a result, the twins can develop reverse asymmetric features. This term is not really a type of twin, just an 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% of identical twins are mirror image twins. Isn’t that interesting?
7. Parasitic Twins
A type of conjoined twins that develops asymmetrically, with a smaller, less formed twin dependent on the stronger, larger twin.
8. Semi-Identical Twins
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 intersex being raised as a female, with both testicular and ovarian structures, while the other is anatomically male.
9. Twins with Different Birthdays
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.
10. Twins of Different Races
Just take the heteropaternal superfecundation idea, then with sperm owners of different ethnicities or “races”.
Wonder why twins would feel pathologized? More on this later…



