If you didn't know, there are two types of twins - identical and fraternal. Identical twins are also called monozygotic, meaning one egg. They have identical genes and are always of the same sex. Fraternal twins are called dizygotic or two egg. They share approximately 25 percent of their genes, as do any two brothers or sisters.
Identical twins are the result of one egg joining with one sperm to form the zygote.
The zygote then completely splits. This two-cell mass passes down the Fallopian tube and attaches separately or together in the womb. The later the zygote splits after ovulation, that is at 4 to 8 days, the greater the likelihood of the embryos sharing one chorion. If the zygote splits after 8 days, the amnion may also be shared. But if the zygote splits after 14 days, the twins may be joined together at some part of their bodies and are called conjoined or Siamese twins.