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

Conjoined twins are identical twins who are joined together somewhere in the body. Conjoined twins are monozygotic twins, in the sense that they share the same zygote. Sometimes, they also share some vital internal organs. Formation of conjoined twins is believed to be the result of late twinning. When the twinning occurs more than twelve days after fertilization of the egg, it may lead to formation of conjoined twins because the embryo may not split completely. This may be due to genetic or environmental factors.

Conjoined twins are also known as Siamese twins, named after the famous conjoined twins, Eng and Chang Bunker from Siam. Conjoined twin births are very rare, amounting to around one birth in every 1,00,000 births. Mary and Eliza Chulkhurst, famously referred to as the biddenden maids, are one of the earliest known set of conjoined twins. The ratio of male and female sets of conjoined twins was found to be three to one, and they are found more in certain countries like India or Africa.

Conjoined twins rarely survive because of the complexity of the bodies. Most of them are stillborn or die within twenty-four hours after birth. Conjoined twins can be separated surgically if none of the vital organs are involved. There are different kinds of conjoined twins, depending on the part of the body where they are joined. These are cephalopagus, craniopagus, craniothoracopagus, dicephalus, iscopagus, omphalopagus, parapagus, pygopagus and thoracopagus. There are also some rare kinds of conjoined twins, such as parasitic twins (where one twin is not completely formed and depends on the other twin to sustain life), and fetus in fetu (where one twins fetus is present inside the body of the other twin).

Some people consider separation of conjoined twins as unethical if it involves death or disability of one of the twins. Conjoined twins, if they survive, can lead healthy lives. There are instances where conjoined twins have even married and become parents. The famous Siamese twins, Chang and Eng Bunker, fathered twenty-one children in thirty-one years.