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There are two types of twins:
Identical
Identical twins (also called monozygotic) occur when a single egg is fertilised and then splits in half. These twins will be the same sex and will usually look very similar to each other. They have the same DNA or genetic material as each other.
If the egg splits within the first three days after fertilisation then the twins will be dichorionic, this means that they will each have their own sac with seperate placentas.
If the egg splits between three and nine days after fertilisation then the twins will be monochorionic and diamniotic, which means that they will share a placenta but still be in two seperate sacs.
If the egg splits between nine and twelve days after fertilisation the the twins will be monochorionic and monoamniotic, meaning that they will share a sac and a placenta.
If the egg splits after twelve days there is a chance that the twins could be conjoined (siamese).
Non-Identical
Non-Identical twins (also called dizygotic or fraternal)are much more common than identical twins. They occur when two seperate eggs are fertilised by two seperate sperms. One half of non-identical twins result in boy=girl twins.
There are many theories as to why twins occur but despite a huge amount of research, the exact cause is unknown. However, non-identical twins are more likely to occur in women that:
Are above thirty
Are of African origin
Have a history of twins in their family
Have children already
have good nutrition
Have increased fertility
Have undergone fertility treatment
Have used the contraceptive pill
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