Conjoined twins are identical twins in which the two embryos fail to separate completely before birth. It is thought that the condition outcomes from incomplete splitting after the twelfth day of fetal development. If a fertilized human egg divides into two embryos before the twelfth day, the identical twins will be born normally as two separate infants. The longer the delay in the separation of the two embryos, the more complicated the connections between the conjoined twins are likely to be. Conjoined twins could be categorized in several different ways.
One distinction that is often made is between symmetrical (equal) conjoined twins, in which both infants are well developed; and asymmetrical (unequal) conjoined twins, in which an incomplete twin is joined to a fully developed sibling. Conjoined twins are also classified according to the points at which their bodies are joined. This system goes back to a French biologist named Étienne Geoffroy Saint-Hillaire (1772-1844), who based his terms on the formal medical words for parts of the body.
Conjoined twins may share tissue, organ systems, or both, depending on the points at which their bodies are fused:
Estimates vary somewhat, but most researchers think that conjoined twins occur once in every 33,000-165,000 births; nevertheless, 40-60 percent of conjoined twins are stillbirths. In the United States, one in every 200,000 live births is really a set of conjoined twins. Male conjoined twins are more likely to be stillbirths. Conjoined twins that are born alive are 3 times more likely to be females. Conjoined twins are more common in India and Africa than in Europe or North America. The factors for this distinction aren't recognized.
Identical twins develop when a fertilized human egg splits into two separate embryos during the first twelve days following conception. If the split occurs after the twelfth day, the twins will not separate fully from each other and will probably be conjoined at birth. The trigger of most instances of delayed splitting isn't known; no genes have yet been identified that lead to conjoined twins. There have been, nevertheless, about ten cases reported in which conjoined twinning occurred after the mother had been exposed to a drug given to stimulate ovulation or a drug utilized to treat fungal infections.
The symptoms of conjoined twins depend on the location of the fusion between the two twins and regardless of whether the twins are symmetrical (equal) or not. In many instances the twins die shortly after birth from heart failure, lung failure, or obstruction in the digestive tract.
In some cases the physician might suspect the presence of conjoined twins early in the pregnancy by finding that the mother's uterus is larger than expected and hearing two fetal heartbeats when listening through a stethoscope. Conjoined twins could be diagnosed by ultrasound as early as the eighth week of pregnancy. Magnetic resonance imaging (MRI) can be used to identify which organ systems are shared between the twins. Computed tomography (CT) scans are primarily helpful in evaluating the bony structures of twins fused at the hips or pelvis. Electrocardiograms (ECGs) and electroencephalograms (EEGs) may be utilized to evaluate the extent of shared heart or brain function and to determine regardless of whether surgical separation of the twins is possible.
The treatment of conjoined twins is highly individualized. It nearly usually requires complicated surgery in one of a little number of medical centers (3 in the United States, one in the United Kingdom, one in New Zealand, and one in South Africa). Doctors classify conjoined twins for treatment in one of 3 categories: those who will die shortly after birth; those who need immediate emergency surgery; and those who will survive until they are old enough for surgery to have a higher chance of success (usually six to twelve months). Twins who share a heart or brain usually can't be separated with out causing the death of both twins.
Conjoined twins are generally delivered by cesarean section instead of waiting for the mother's due date. The operation is usually scheduled for two to four weeks before the due date. Surgery to separate the twins may be performed instantly after delivery if one or both twins have a life-threatening emergency. These operations are complicated and may take as long as thirty-five hours to complete. Two complete surgical teams are required to care for the twins after the separation is complete. In most cases, conjoined twins who survive separation will need further surgery at intervals during childhood.
The prognosis of conjoined twins is often poor. Between 40 and 80 percent of twins who need emergency surgery after birth die in intensive care following the operation. In some cases, particularly those involving parasitic twins, the parents must make the painful decision to allow one of the twins to die if the other would be to have any chance of survival. Conjoined twins who are healthy enough to have separation surgery postponed until they're older have a survival rate of 80 percent.
It's possible for conjoined twins who aren't separated to have productive and satisfying lives. One set of conjoined twins in Minnesota completed high school in 2008 and obtained a driver's license. One twin in a set of conjoined sisters has made a career as a country music singer. Conjoined twins who are not separated, however, have shortened life expectancies; most pairs die in their twenties or early thirties. As of 2008, the oldest recognized living set of conjoined twins was a pair of brothers in Ohio born in 1951.
There's no known way to prevent conjoined twins other than avoidance of the small number of drugs that have been associated with conjoined twinning in a few instances. The condition is considered to be sporadic (occurring at random); parents don't have an increased risk of having a second set of conjoined twins in a later pregnancy.
The widespread use of ultrasound and other imaging methods has led to improved prenatal identification and diagnosis of conjoined twins. It is feasible that the greater numbers of conjoined twins that are identified before birth will help researchers understand the causes of this type of twinning and discover better methods of surgical separation and treatment for conjoined twins.
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