Twin Types

There are different types of twins. They are broadly classified as:

1. Fraternal Twins
2. Identical Twins
3. Siamese Twins
4. Mirror Twins

Fraternal Twins are the most common type of twins. They are the result of the union of two eggs and two sperm. They may not look alike and can be different sexes. Fraternal twins can be conceived at separate times and can have different fathers. The tendency to conceive fraternal twins seems to be heredity. Some studies show higher incidence where there is a history of twins on the mothers' side of the family. About two-thirds of all twins are fraternal.

Opposite-sex twins make up one-third of the twin population, about one in 240 births. Boy/girl twins develop as differently as single birth children from the time of conception. Boys are usually heavier and longer than girls both before and after birth. Growth is faster in boys than girls until about 7 months of age, at which time females begin to grow faster until about the age of 4 years. Girls, on the average, tend to sit, crawl and walk earlier than boys. Boys show earlier visual skills while girls show greater talking skills.

While Fraternal twins do not usually develop the closeness and psychological interdependence that identical twins do, they do develop that " special bond" that all twins are known for.

Identical Twins happen when a single fertilized egg splits usually one to fourteen days after conception. Identical twins are genetically alike. They have the same chromosomes and usually have a lot of the same physical features but are not totally the same. Each twin is an individual and has different traits of his own. Identical twins are the same sex, the same blood type and have the same hair and eye color, same nose, ear and lip shapes.

The identical birth rate is approximately 4/1000 births and is thought to be an accident of nature. Chances of having two sets of identical twins is one in 70,000.

Identical twins spend a lot of time together and are usually each other's best friend. Although they like other people, their twin is the person they usually choose to be with, especially when they are young. They do not like to be apart. Identical usually have their own private language and have the ability to do the same in the same activities. They also enjoy each others company because they usually have the same interests.

Identical twins usually care for one another more deeply than fraternal. This is because of the special bond between them that grew while they were inside their mother. After age 9 or 10, identical usually choose to form their own identities.

Siamese twins are also called "Conjoined Twins" and are born joined at identical sites. They are very rare. Siamese twins' result when the fertilized egg fails to divide completely, as it does with most identical twins. often, one twin does not develop and may appear to be an extra limb or even a tumor on the healthier twin. joined twins are always identical. Surgical separation is often possible. an operation was successfully performed in 1953 on twins who were joined near the base of the spine and shared the lower intestinal tract, and in 1979 on twins joined at the skull.

Siamese twins occur 1 in 50,000 to 1 in 100,000 births, and three times more likely to be girls than boys. Siamese twins joined at the head is the rarest form and occurs about 1 in 2.5 million births. Twins that are joined at the chest is the most common form and is found in 70% of Siamese twins.

Almost one quarter of identical twins are "mirror twins". This means that the twins will appear to be reflections of one another. One twin will be right-handed and the other will be left-handed. Internal organs and skeletal features will be on the opposite sides of the body. And, hair whorls will turn in opposite directions. One in four identical twins are mirror-image. Siamese twins show a high degree of mirror imaging. Mirroring is just as much a mystery to medical science as the cause of identical twins.