Twins are a kind of multiple births when a woman gives birth to more than one baby at the same time. Twins are formed when more than one egg is fertilized or when the same egg is fertilized by one or two sperm, leading to the formation of more than one fetus

Twins can be identical or fraternal, and sometimes there are cousins who look like twins. But Siamese twins are a unique and unusual type of twins. Conjoined twins term is for two babies who are physically connected to each other from birth. Conjoined twins develop when an early embryo only partially separates to form two individuals. Although two fetuses develop from this embryo and they remain physically connected most often at the chest, abdomen or pelvis. Conjoined twins may share more than one internal organ.

The Term Siamese Twins Origin

The term Siamese twins came from Chang and Eng Bunker who were the first pair of conjoined twins to become internationally known. The brothers were from Siam, hence they were known as The Chinese Twins. In 1829, Robert Hunter arranged a world tour for the twins, exhibiting them as circus freaks, and calling them as United Brothers. Later in the 19th century, Siamese twins became a general term for conjoined twins.

How twins are joined

Conjoined twins are typically classified according to their joined parts. They sometimes share organs or other parts of their bodies. Some common joined parts are:

  • Chest: They share a heart and may share a liver and upper intestine. It is one of the most common sites of conjoined twins.
  • Abdomen: In this, they share the liver, and some share the lower part of the small intestine, but they do not share a heart.
  • Head: They share a portion of the skull. But their brains are usually separate, though they may share some brain tissue.
  • Trunk: Twins are joined side to side at the pelvis and part or all of the abdomen and chest, but with separate heads.


After conception, within 8 – 12 days the embryonic layers split to form monozygotic twins and develop specific organs and structures. But when the embryo splits between 13 and 15 days after conception, then separation stops before completion, and the resulting in conjoined twins. An alternative cause is that when two separate embryos may somehow fuse together in early development.


There are no specific symptoms that indicate a conjoined twin pregnancy. As other twin pregnancies, the uterus grows faster than with a single fetus, and there will be more fatigue, nausea, and vomiting in the early pregnancy. Conjoined twins can be diagnosed early in the pregnancy using standard ultrasound.


Conjoined twins are delivered by cesarean section. Many conjoined twins die shortly after birth, and some surviving twins undergo separation surgery. The success of surgery depends on what causes Siamese twins, how they are joined and which organs they share, as well as the experience and skill of the surgical team.

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