Identical twins are not hereditary. Identical twins form from one fertilised egg (a zygote), which splits into two embryos. As of yet, there is no scientific research that has been able to determine why this occurs. Identical twins share the same DNA.
It is the timing of the split that determines the type of twins that are formed.
- If the zygote splits within 3 days of forming, then each baby will each have their own amniotic sac and their own placenta. These twins are known as Dichorionic/Diamniotic (Di/Di twins).
- If the zygote splits between 4-9 days (blastocyst stage) both babies will share the placenta, however, will have separate sacs. These twins are known as Monochorionic-Diamniotic twins (Mo/Di twins).
- If the split occurs between 9-13 days then the babies will share one placenta and one sack. These twins are known as Monochorionic-Monoamniotic (or Mo/Mo twins). This type of pregnancy will require a very high level of monitoring as it is very easy for the babies cords to become entangled.
- If the split occurs between days 13-15 then it will mean that the developing embryo will not fully split. These twins are known as Conjoined twins. Thankfully this is very rare.
Fraternal twins mean that each baby has its own placenta and own amniotic sac. These twins are known as twins are known as Dichorionic/Diamniotic (Di/Di twins). Fraternal twins have their own DNA and are basically siblings who share a womb.
Typically a woman will only release one egg during ovulation. However, sometimes a woman will release more than one egg. This can be due to genetics, maternal age, ovulation-inducing drugs, ethnicity or just pure luck!