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Twins
The Science Behind Those Dynamic Duos
By Carma Haley Shoemaker
In addition to fraternal and identical twins, other types of twins may present themselves as well. These types are conjoined and unequal twins.
Conjoined twins are identical twins who did not completely separate during fetal development, and the offspring remain attached to each other and may even share body parts or organs.
"What happens with conjoined twins is that the egg does not separate completely," says Hanson. "It is the degree of separation that determines how much they share ultimately. They can be joined at the head, or more commonly, the abdomen or pelvis. Sometimes they share legs; sometimes they can share liver, heart, kidneys or other internal organs. Sometimes they can be easily separated, and sometimes it can be a very complicated operation. However, there are conjoined twins who have never been separated and have grown and developed into healthy, well-adjusted adults."
Unequal twins are two separate fetuses born of the same pregnancy in which only one of the pair is fully formed. The other fetus shows varying degrees of developmental defects. The differences that characterize unequal twins do not have to be that of a serious nature, and it may only be that one of the pair is larger by a minimum of 2 pounds or 1 inch.
"Size is the major factor with unequal twins," says Hanson. "However, in most cases, one of the pair will suffer from several physical problems, such as underdeveloped lungs or heart. It is thought that unequal twins result from an unequal separation of the ova during development, which leads to one of the pair receiving more nutrients, oxygen and room in the uterus. Unequal twins are rare and make up only about 2 percent of all multiple births."
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