Conjoined twins are a rare physical malformation that develops during pregnancy. The result of the disorder is that two fetus siblings are physically joined in some fashion and are born this way.
In a typical pregnancy, an egg is fertilized by a sperm to form a zygote, which eventually grows into a baby. When twins occur, they are either identical, in which one zygote splits in two to form two babies, or fraternal, when two separate eggs are fertilized to form two zygotes. With conjoined twins, a zygote tries to divide but does not divide completely. The two babies then grow and develop in the womb joined to one another.
Conjoined twins are an extremely rare occurrence. Though twins in general represent about one in every 90 births, conjoined twins occur in one of every 50,000 to 200,000 births.
Types and Complications of Conjoined Twins
There are different types of conjoined twins, classified by the areas of the body where they are joined. These areas, along with the extent to which the twins are joined, play a large role in whether the twins will be able to be separated and whether one or both will ultimately survive.
Conjoined twins pose a number of challenges. Depending on where they are joined, the twins may share some vital internal organs, and other organs may be underdeveloped or misshapen because of the way they grew in the womb. This can include the intestines, heart, liver and urinary system.
Outlook for Conjoined Twins
Treatment of conjoined twins is challenging, but sometimes successful. About 40 to 60 percent of conjoined twins are stillborn, and 35 percent of successfully delivered conjoined twins do not survive the first 24 hours. However, advances and successes have been seen more frequently over the past 20 years, and the number of conjoined twins who have been successfully separated is increasing.
SOURCES: American Pediatric Surgical Association; Mayo Foundation for Medical Education and Research