Conjoined twins are an extremely rare type of identical twins who are physically joined at birth.
Scientists believe conjoined twins form because of a delay in the fertilized egg's division. In normal identical twins, the egg splits at four to eight days after fertilization. In conjoined twins, however, the split occurs sometime after day 13. Instead of forming two separate embryos, the twins remain partially attached as they develop inside the womb. In most cases, conjoined twins do not survive more than a few days past birth because of a high rate of malformed organs and other severe birth abnormalities. However, surgical separations have been successful in conjoined twins that have a superficial physical connection.
Conjoined twins are commonly referred to as Siamese twins, although this is now considered a derogatory term. The phrase Siamese twins originated from the famous conjoined twins Eng and Chang Bunker, who were born in Siam (Thailand) in 1811.
Some conjoined twins are attached at the upper body, others may be joined at the waist and share a pair of legs. Conjoined twins often share major organs such as a heart, liver, or brain. Medical experts have identified several types of conjoined twins. They are classified according to the place their bodies are joined. Most of the terms contain the word pagus, which means "fastened" in Greek.
Cephalopagus: A rare form that involves conjoined twins with fused upper bodies and two faces on opposite sides of a single head.
Craniopagus: Conjoined twins with separate bodies and one shared head is a rare type and only occurs in 2% of cases.
Thoracopagus: About 35% of conjoined twin births have this common form of the condition, which joins the upper bodies. These twins usually share a heart, making surgical separation nearly impossible.
Ischopagus: About 6% of conjoined twins are attached at the lower half of the body.
Omphalopagus: The type of conjoined twins that are attached at the abdomen and that often share a liver accounts for approximately 30% of all cases.
Parapagus: About 5% of conjoined twins are joined along the side of their lower bodies.
Pygopagus: About 19% of conjoined twins are joined back to back with fused buttocks.
Dicephalus: Twins that share one body, but have two separate heads and necks.
Parasitic twins: This occurs when one smaller, malformed twin is dependent on the larger, stronger twin for survival.
Fetus in fetu: In this unusual case, one fetus grows inside the body of the other twin.
Scientists are still searching for the cause of conjoined twins. They believe a combination of genetic and environmental factors may be responsible for this rare condition.
Conjoined twins occur in one out of every 50,000 births. Many such pregnancies are terminated before birth, or the infants are stillborn. Conjoined twins are always identical and of the same sex. They are more often female than male, by a ratio of 3:1. Conjoined twins are more likely to occur in Africa, India, or China than in the United States. Conjoined twins have appeared in triplet and quadruplet births, but no cases of conjoined triplets or quadruplets have ever been reported. Most parents of conjoined twins are younger than 35 years old.
Signs and symptoms
Approximately 50% of women who are pregnant with conjoined twins will develop excess fluid surrounding the fetuses, which can lead to premature labor and an increased risk of miscarriage. Conjoined twins joined at the abdomen (omphalopagus) are more likely to be breech babies. In breech births, infants are born feet or buttocks first instead of head first. Most omphalopagus conjoined twins are born by cesarean section to increase their odds of survival.
Conjoined twins can be born with a complication called hydrops, which causes excessive fluid to build up in an infant's body and can be life-threatening. Those who survive past birth may experience congenital heart disease, liver or kidney disease, physical or mental disabilities, and intestinal blockages.
Physicians typically try to determine if a woman is having conjoined twins at an early stage so that the parents can have an option to terminate the pregnancy if the odds of survival are low. Ultrasound imaging is a technique in which high-frequency sound waves create a picture of a developing fetus inside the womb and is often used to make the diagnosis. Initial diagnosis is possible at 10-12 weeks of gestation, but it is difficult to determine which body structures are involved until 20 weeks of gestation.
In utero, the three-dimensional magnetic resonance imaging (MRI) test is another important diagnostic tool that helps more precisely define which body parts of the conjoined twins are connected. An abdominal x ray of the mother is used to look for connected bones in conjoined twin embryos.
Treatment and management
Early diagnosis is key so that families and healthcare providers can begin to plan for the birth of conjoined twins. Because of the high rate of miscarriage and difficult labor, most conjoined twins are delivered by cesarean section. Some conjoined twins have survived and lived full lives without serious medical interventions. If the twins do not share a large number of organs, however, physicians typically will recommend a surgical separation.
A large medical team must be assembled for a surgical separation. Physicians prefer to wait for a few months after birth, but that may not be possible if the twins are born with life-threatening congenital abnormalities. The type of surgery that is performed is determined by where the twins are connected. Doctors will often insert tissue expansion devices into the twins' skin before the operation to promote better healing at the site of separation.
Conjoined twins who survive a surgical separation will have many ongoing health-care needs, from wound care to prosthetic limbs and special diets. As the twins grow up and start school, they also may need counseling to help them adjust.
The majority of conjoined twin pregnancies are not successful. However, most conjoined twins who undergo a planned surgical separation several months after birth do survive. The survival rate for conjoined twins who need an emergency separation at birth is approximately 44%.
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Center for Loss in Multiple Birth, Inc. (CLIMB). PO Box 1064, Palmer, AK 99654. (907) 222-5321.
Center for Study of Multiple Birth. 334 E. Superior St., Suite 464, Chicago, IL 60611. (312) 266-9093. <http://www.multiplebirth.com>.
Conjoined Twins International. PO Box 10895, Prescott, AZ 86304-0895.
National Organization of Mothers of Twins Clubs. PO Box 438, Thompson Station, TN 37179. (615) 595-0936. <http://www.nomotc.org>.
Twins Foundation. PO Box 6043, Providence, RI 02940-6043. (401) 751-8946. [email protected]
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