Causes of identical twins
The different type of twin pregnancy’s depends a lot on what happens with the placenta.
Identical twins have either their own placenta or share a placenta.
The reason for there being either one shared placenta or one placenta for each twin is not known, however, the later the embryo splits following after fertilisation happens the more the complications.
- Embryo splits up to 4 after conception – result is separate placentas called dichorionic, just like fraternal twins. This type of twin pregnancy has the lowest rate of complications. Have the lowest risk of mortality of all twin pregnancy’s at about 9%, although this is still significantly higher than the mortality rate of singletons.
- Embryo splits between 4 to 8 days after conception – leads to shared placenta, called monochorionic, but will have separate sacs called diamionic. This type of twin pregnancy is the most common placenta type for identical twins. If they are monochorionic they are definitely identical twins. Diamionic twin pregnancy’s have a 25% mortality due to twin-twin transfusion.
- Embryo splits over 8 days to about 12 days– leads to monochorionic twins but within the same sac called monoamnionic. Monoamnionic monochorionic twin pregnancies have the second highest mortality rates, at about 60% due to cord entanglement prior to 32 weeks. The majority of monoamniotic monochorionic twin pregnancies are delivered by caesarean at 32 weeks. << Read more about Monoamnionic monochorionic >>
- After 12 days – the splitting results in conjoined twins. The mortality rate is highest for conjoined twins, due to the complications of sharing internal organs.
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