Conjoined twins are two separate bodies that fused together during development. They can be asymmetric or identical. They may also share organs and structures. They can live as single individuals, or they can be separated surgically.

Often, conjoined twins are born when an embryo that should split into two identical twins doesn’t completely separate. Scientists aren’t sure why this happens.

Ultrasound

When doctors discover conjoined twins in a pregnancy, they usually make the diagnosis through routine ultrasound in the first trimester. Detailed ultrasounds and tests that use sound waves to produce images of the hearts (echocardiograms) can help determine the extent of the twins’ connection and how their organs function.

Conjoined twins are very rare. They occur when an egg becomes fertilized by sperm and then splits to create two identical twin embryos. Identical twins are always the same sex, and most of them develop into healthy babies when they’re born. However, in some cases, the process of separation fails and the twins remain fused together. These are called conjoined twins, or Siamese twins. It’s not clear why the process of separation fails.

Some people don’t even know they have conjoined twins because the condition is so rare. Others face serious complications during the pregnancy and after birth. Most of these babies die during the pregnancy or die shortly after birth, but a few survive and are surgically separated.

The most common type of conjoined twins is thoracopagus, which means they’re joined at the chest. These twins can be back-to-back or facing each other, and may share the lungs, the upper digestive tract, and the genital and urinary systems. They might also have two or three legs. Another type is ischiopagus, which means the twins are joined at the pelvis. They can be face to face or back-to-back and might share the liver, biliary tract, and parts of the lower digestive tract. These twins might have two or three legs, and their brains are sometimes connected.

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MRI

When an egg becomes fertilized by sperm, it should divide into two separate embryos. But in some cases, these twins don’t fully separate during development and remain fused until birth. There are many theories as to why this occurs, but more research is needed to understand conjoined twins.

Conjoined twins are rare and have fascinated people for centuries. They have been seen in cave drawings, pottery, and figurines from prehistoric times. The famous Siamese twins, Chang and Eng Bunker, were joined at their xiphisternum and became the model for the term “Siamese” when they were born in 1811. The Mutter Museum in Philadelphia has plaster casts of their livers, and the Natural History Museum of Vienna has a skeleton of Janiceps, another pair of twins who were joined at the head.

In utero, a diagnosis of conjoined twins is possible by ultrasound and magnetic resonance imaging (MRI). MRI avoids ionizing radiation and allows for detailed evaluation of visceral fusion and fetal anatomy. The most common site of fusion is the thoracopagus, in which twins are joined at the chest (sternum and diaphragm) and upper abdominal wall. These twins also share the liver and a common pericardium and heart. In less than 5% of conjoined twins, the babies are joined at the pelvis (ischiopagus) or in the genital area (pygopagus). The least common type of conjoined twins is craniopagus, in which the skulls are fused together.

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Pregnancy tests

If you have twins, your doctor will determine whether they are identical (monozygotic) or fraternal (dizygotic). Identical twins form from a single fertilized egg that splits. Fraternal twins form from two separate sperm/egg combinations. You can also have conjoined twins, which are not the same sex and are joined at specific sites on the body, such as the thorax, abdomen (omphalopagus), sacrum (pygopagus), pelvis (ischiopagus), or skull/face (cephalopagus).

It is not clear why conjoined twins occur. They may result from incomplete separation of an early embryo. Alternatively, the twins may form from a single fertilized egg that remains connected to another embryo, or the twins could develop from two separate eggs that fuse during fetal development.

In most cases, doctors discover conjoined twins through an ultrasound in the first trimester of pregnancy. They can then conduct follow-up tests to learn more about the twins’ condition and how they are connected.

The most common type of conjoined twins are thoracopagus, which are connected at the thorax and abdominal wall. They often share a liver and intestines. Pygopagus twins are joined at the buttocks and perineum, and may share a long portion of the sacral spinal canal. Ischiopagus twins are joined at a bony pelvis, and they may have four normal legs or two legs that are fused into one malformed limb.

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Follow-up tests

If you have conjoined twins, follow-up tests can help determine whether they have sex. These tests can include ultrasounds and MRIs, which can reveal the presence of shared organs and other abnormalities. A doctor can also advise expectant parents on the best course of action for their pregnancy.

The rare condition of conjoined twins is the result of fusion or splitting of embryonic cells early in the process of fertilization. It is estimated that conjoined twins occur in about 50,000 pregnancies per year, but the majority are stillborn or die shortly after birth. Those that survive are typically identical and more often female.

There are eight types of conjoined twins. They are named for their site of union and may have four (tetrapus), three, or two legs. The most common type of conjoined twins is thoracopagus, which are joined at the chest and abdomen. Other types of conjoined twins are omphalopagus, pygopagus, ischiopagus, rachiopagus, and cephalopagus.

The most famous pair of conjoined twins are Marta and Milagro Juarez, who were born in Peru in 2000. Their doctors performed an MRI before surgery to separate them. It revealed that the twins had a single head, fused thoraces and abdominal walls, two separate spinal columns, a shared liver and kidney, and one heart. They also had a shared pulmonary artery and shared lymph nodes.