When bearing a child there may be a possibility of having more than just one child, which is referred to as a multiple birth. A multiple birth is one in which two or more fetuses develop at one time in the uterus. This happens when a woman naturally releases more than one egg from her ovaries. Most multiple births are caused from using fertility drugs.
The most common types of multiple births are twins. Twins are categorized into different groups. The two most common types of twins are fraternal and identical.
Fraternal twins are a result of two eggs and two sperm (Segal 4). Twins can be conceived at different times when there are two separate sperm. The tendency to conceive fraternal twins seems to be heredity. Some studies show higher incidence where there is a history of twins on the mother's side of the family. Fraternal twins can be boy/girl, girl/girl, or boy/boy. Opposite sex twins make up one-third of the twin population, about one in 240 births (Wolner 32). Boy/girl twins develop as differently as single birth children from the time of conception. Fraternal twins can be conceived at the same time, but their birthdays can be days, weeks or even months apart (Segal 37).
The second most common type of twins is identical twins. Chances of having identical twins are one in two hundred births (Vaughan 12). Identical twins happen when a single fertilized egg splits usually one to fourteen days after conception (Segal 6). Identical twins are genetically alike. They have the same chromosomes and usually have a lot of the same physical features. They always have the same blood type, hair, eye color, nose, ears, and lip shapes .
Almost one quarter of identical twins are mirror twins. This means that the twins will appear to be reflections of one another (Wolner 102). 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.
