The twins originate from a single fertilised egg, so they are always identical and of the same sex.

BBC: NEWS | Health | Medical notes | Conjoined twins

Conjoined twins are created just a few days after they are conceived - most likely by the incomplete splitting of the fertilised egg.

BBC: Twin separations: A history of risk

Putting only one fertilised egg into the womb avoids multiple births, which are known to increase the health risks of both mother and child.

BBC: Non-invasive embryo gene screen

Some have suggested even tighter rules which would see only a single fertilised egg placed in the womb, as is the case in Scandinavia.

BBC: IVF under the microscope

Since the 19th century, biologists have been fascinated by the relationship between ontogeny (the development of an adult from a fertilised egg) and evolution.

ECONOMIST: The first complete DNA sequence of an animal is now in the bag

Dan and Scott Shillum are identical twins, genetic clones produced from the accidental division of a single fertilised egg into two embryos in their mother's womb some 40 years ago.

BBC: 'Not-so-identical' twins may hold the key to disease

Put on the ballot by a home-schooled Baptist woman, this would define a fertilised human egg as a person.

ECONOMIST: Ballot initiatives

Once an egg has been fertilised, it begins dividing.

ECONOMIST: A survey of the human genome

Building on earlier research, which showed that egg cells from rabbits could be fertilised in test tubes when sperm was added, Edwards developed the same technique for humans.

BBC: Test-tube baby pioneer Sir Robert Edwards dies