Conjoined Twins

Definition - What does Conjoined Twins mean?

Conjoined twins are identical twins who are connected together at some point on their bodies. They are very rare, taking place in about 1 in 50,000 pregnancies. They have a low survival rate due to the complications that can result from sharing organs and other body parts. Surgery is often used to separate conjoined twins after they are born. There is no known genetic cause of conjoined twins.

FertilitySmarts explains Conjoined Twins

Conjoined twins are created when, after one egg is fertilized by one sperm, the resulting zygote begins to split. Splitting of a zygote after fertilization usually results in identical twins of different types, some sharing placentas and/or amniotic sacs. Conjoined twins are a variety of monochorionic-monoamniotic twins, sharing both a placenta and an amniotic sac. In this case, the zygote begins to split 13 or more days after fertilization but does not separate completely. About half of conjoined twins are stillborn, and another 35% die within a day of being born. For no known reason, more male conjoined twins are created in the uterus, but most successful births of conjoined twins are female. Conjoined twin pregnancies can be detected by ultrasound late in the first trimester.

Conjoined twins are delivered at least 2-4 weeks early, always by C-section. There are many types of conjoined twins, categorized by where they are joined. They are assessed shortly after birth to see if they can be separated surgically. Doctors can use MRI and ultrasound to determine which organs the twins might share and classify the degree of difficulty of the surgery. If a vital organ or body part is shared, the surgery is riskier. In many cases, one or both of the twins may not survive any attempts to separate them, making the cases controversial. Specialists and parents have to weigh the quality of life for the twins against their likelihood of survival. If surgery is chosen, it is done when the twins are 2-4 months old, unless emergency separation is required.

Conjoined twins were once referred to as “Siamese twins” because of a famous pair of conjoined twins born in 1811 in what was then Siam (now known as Thailand). This is not considered a proper term anymore as conjoined twins have been born all around the world to a variety of racial and ethnic groups.

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