Wednesday, July 14, 1999 Published at 19:10 GMT 20:10 UK
Twins occur in one of every 90 births
One in 12 people are conceived as twins, although many do not even know it.
In most cases, the twin dies in the womb or shortly after birth.
According to BBC One's series, The Secret Life of Twins, nearly a third of twins are born dangerously premature.
This means they are not fully developed and have a lower chance of survival.
The first episode of the series looks at identical twins, who share the same genes and are the result of a single embryo splitting.
Usually, this happens in the first week and results in twins who look like carbon copies of each other.
But if it occurs between one and two weeks, it results in "mirror twins". One, for example, may be right-handed and the other left-handed.
If separation does not happen for two weeks, the two parts cannot develop completely, resulting in twins who are physically joined together.
The original Siamese twins were Eng and Chang Bunker, who came to the US from Siam in 1829.
They were joined at the stomach by a narrow strip of liver and could be easily separated today.
Both are reported to have had very separate characters.
Chang was said to be the more intelligent twin, but was bad-tempered and an alcoholic.
Eng did not drink and was more easy-going.
Research shows that twins who are joined even at the brain can have totally separate personalities.
But sharing the same blood supply has some consequences.
For example, if one twin gets drunk, the other feels the effects, if not to the same degree.
Traits, such as blackheads, spots or skin cancer, can lie dormant in identical twins for years and then appear at more or less the same time in each person.
They face regular scans.
Because their arteries are connected and they share the same blood supply, one twin often manages to get more blood and nourishment than the other.
Professor Sir Robert Winston, presenter of the twins series and a fertility expert, said: "One is literally sucking the life out of the other."
If the twins are in the later stage of pregnancy, the doctor will usually suggest a Caesarian Section.
Professor Sir Robert says twins are endlessly fascinating because they help to explain what makes us what we are.
The Secret Life of Twins is on BBC One at 9.30pm BST on 14 July 1999.