Multiple births are rare in humans. Twins are the most common form of a multiple pregnancy. About one in eighty-sevens births result in twins. Most of the time, twins are found as fraternal or identical. Siamese, polar body, and sesquizygotic twins are very rare types of twins. The total number of births of twins remains fairly constant, but the birth of fraternal twins alter greatly. The determination of the type of twins is based on how the embryos develop.
The birth of twins can occur in two different ways, the fertilization of a single egg or the fertilization of two eggs. In the case of dizygotic twins, the woman’s ovaries release two eggs at the same time, with each being fertilized by two male sperms, and fraternal twins begin to develop. Each fetus lies within its own membrane in two separate sacs, with two separate placentas. The two zygotes do not share blood vessels. Both zygotes develop differently, each having a different genetic coding. They can be both girls, both boys, or a single girl and a single boy. Because each embryo develops on its own from different genetic characteristics, dizygotic twins resemble family characteristics, as do brother and sister. Dizygotic twins share on average about 50% of the same genetic material.
Dizygotic Twins Diagram

The births of monozygotic twins take place much differently than the birth of fraternal twins. Identical twins originate from a single egg, fertilized similarly to a single pregnancy. A change transpires in the pregnancy that causes the development of identical twins. The change from a single birth to the birth of monozygotic twins occurs when the zygote ruptures into two separate structures. These two parts begin to develop into individual fetuses, sharing a similar genetic code and developing in a similar manner. Monozygotic twins share 100% of their genetic material. Identical twins resemble each other very closely and have similar fingerprints and blood types.
Monozygotic Twins Diagram Siamese twins, also known as conjoined twins, develop in a similar fashion as monozygotic twins. Siamese twins develop when an early embryo partially seperates to form two individuals. Two fetuses will develop, but they will physically remain connected. Such twins are usually joined at the hip, chest, abdomen, buttock, or head. With current monitoring equipment, conjoined twins can be detected maturing in the mother, and during a Cesarean section is sometimes needed to deliver the children safely. Seperation of the twins sometimes leads to the death of one or both twins. These births are a rare event, occurring only about once in every fifty-thousand births.
Siamese Twins Diagram Polar body twins and sesquizygotic twins are two types of twins that are very rare. Polar body twins, known as half-identical twins, develop when an unfertilized ovum splits into two but equal parts and is subsequently fertilized by two separate sperms. These types of twins are less than identical but more than fraternal. Half-identical twins share about 75% of genetic material. Sesquizygotic twins are developed when two sperm fertilize a single egg, forming a triploid, which then split. Sesquizygotic twins, also known as semi-identical twins, are distinct from polar body twins because the egg splits after fertilization rather than before. At this time, these rare hybrid types of twinning are very difficult to identify and confirm without access to sophisticated genetic testing technology. Parents may suspect their twins to fit into these categories, but they may never really know for sure.
Polar Body Twins Diagram Sesquizygotic Twins Picture Scientists studying twins are currently debating about the role that genetics plays in relation to environment, also known as the nature versus nurture debate. Twins have been used to prove the greater influence of either nature or nurture on behavior, although more and more studies accept the interplay between the two influences. This is vital for actually understanding the determinants of behavior of different sets of twins. Knowing whether a twin is identical or fraternal is also vitally important for those scientists hoping to explain behavior and development by twin type. The most popular current method of determining whether twins are fraternal or identical is by DNA testing; a swab of cheek cells from each twin will determine whether they are identical or not. Much twin research has focused on the different types of twins, hoping to establish relationships concerning the role that genetics plays. Other social scientists are concentrating on the role that environment plays, but they are now not assuming that twins necessarily share the same environment, any more that any other sibling in a family shares the same environment. In other words, each twin has another person (his or her twin) directly or immediately in their environment, virtually from the moment of conception. And this other person, the twin, modifies and determines the environment in which they both live. They are defined and often treated as twins, although like other siblings they also face varying environments due to gender differences, positions in the family, relation to parents, and so on. Nonetheless, they share the "twin factor."
Twins are different, distinct, special, and fascinating. They have been studied, analyzed, and written about for a long time in all cultures, yet there is still a great deal to learn about twins and from twins.