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Whether you have identical or non-identical twins depends on how many eggs are fertilised and how early on an egg splits.
Dizygotic/Fraternal TwinsDizygotic twins are formed if two of your eggs are fertilised by separate sperms at a similar time and both eggs develop into an embryo. Each twin arises from a separate egg and a separate sperm so the resulting babies are only as genetically similar as normal siblings would be - they are non-identical.
Two thirds of twin pregnancies are dizygotic, with 40% of these being a boy and a girl. Two girls are more common than two boys.
IVF treatment increases the likelihood of having dizygotic twins because it stimulates the ovaries to produce more eggs, so the chance of more than one egg being fertilised increases.
Women who have previously had twins, women above average height and with a BMI over 30 and women older than 35 are more likely to conceive dizygotic twins.
Also certain foods such as cassava are assosciated with an increased chance of conceiving twins because they are thought to stimulate the ovaries.
Monozygotic/Maternal TwinsMonozygotic twins arise from one egg which splits into two shortly after fertilisation. Twins which have developed in this way are 'identical' but the degree to which they are identical varies depending on how late into the pregnancy the egg splits. If the egg splits very early on the resulting twins are less identical than they would be if the egg split a few days later.
Monozygotic twins can be further classified depending on whether or not the two embryos share a placenta (chorion) or an amniotic sac (amnion).
Dichorionic/Diamniotic (di/di) twins each have their own placenta and their own sac. This is the case if the egg splits very early on (within two days of fertilisation). All fraternal twins and 20-30% of monozygotic twins are di/di.
Monochorionic/Diamniotic (mono/di) twins share a placenta but each have a separate sac. The majority of zygotes split after two days, resulting in mono/di twins which are genetically very similar and share and afterbirth.
Monochorionic/Monoamniotic (mono/mono) twins share both a placenta and a sac. This outcome only occurs in 1% of twin pregnancies when the egg splits very late (up to 12 days after fertilisation). Mono/mono twins are always the same sex. There is an increased risk of cord entanglement/compression because the fetuses are wriggling around close to eachother within the same sac.
Conjoined TwinsConjoined twins are monozygotic twins whose egg did not split fully. As a result conjoined twins may share some organs or body parts.
Conjoined twins have a very low survival rate with 35% surviving for just one day. Separating procedures are sometimes an option but they carry high risks.
The most common type of conjoined twins share a heart and are known as thoracopagus. Omphalopagus twins share a liver and gastrointestinal and gastrourinary function. Craniophagus twins are joined at the head and may share parts of the brain.