Some estimates say that one in as many as eighty-thousand of all births results in two babies joined together. Conjoined twins（聯體嬰兒）happen about once out of every two-hundred births of identical twins（同卵雙胞胎）.

Some twins develop from two separate eggs（卵細胞）that are fertilized（受精）at the same time. These babies are called fraternal twins（異卵雙生）. One can be a girl and the other a boy.

Other twins develop from a single egg. These are called identical twins. Identical twins result when a fertilized egg divides into two.

Sometimes the division begins but is not completed. This produces twins who are physically linked. Medical experts say genetic and environmental influences are involved in the development of conjoined twins.

Recent separations have taken place in Britain, Singapore, Italy, Australia and the United States. These included an operation where doctors at the Children's Medical Center in Dallas, Texas, separated two Egyptian boys joined at the head.

Conjoined twins may be linked only by a thin piece of tissue（組織）. Or they may be attached at the chest or other part of the body. Sometimes they share a heart or other internal organ.

In the past, conjoined twins were called Siamese twins（暹羅雙胎）. This was because the first well-known twins were born in Siam（暹羅，泰國的舊稱）, the country now called Thailand. Chang and Eng were born in eighteen-eleven. A twelve-centimeter-long ligament（韌帶）near their breastbones（胸骨）connected them. Chang and Eng grew up, married and had a total of twenty-one children. The two men died in eighteen-seventy-four.

Historically, the survival rate for conjoined babies has been between five and twenty-five percent. But medical progress has increased those chances.

Today, most conjoined twins are found during examinations before they are born. Some are easier to separate than others.

The book "Entwined Lives" by Nancy Segal says doctors have performed about two-hundred operations to separate conjoined twins. Ninety percent have taken place since nineteen-fifty. The book says most of the operations resulted in the survival of at least one of the babies.

One Web site where you can learn more about conjoined twins is twinstuff-dot-com.

This VOA Special English Health Report was written by Nancy Steinbach. This is Phoebe Zimmermann.