|Sadly sometimes one twin does die in the first trimester. This can happen in identical (monochorionic) twins that share a placenta due to early onset twin-twin transfusion syndrome (TTTS), or it can happen in twins where both are separate (non identical, dichorionic). There is a risk (around 30%) in identical twins that share a placenta that one dying could cause the death of the other one, or cause damage to it. In non identical twins, if one dies then the other is not affected. In both scenarios, there is a risk of miscarriage of around 10%. The dead twin cannot be removed as this would disturb and damage the living twin; the dead twin is gradually reabsorbed in its sac and will usually be so small as to be almost invisible on scan by 20 weeks.|
Fraternal (dizygotic) twins are the result of two separate eggs being fertilized by two sperms at the same time, so that two embryos implant and develop with two placentas. The twins may look different and be different sexes.
If an egg is fertilized by one sperm and then splits into two, the two embryos that are produced are always the same sex and are identical because they contain the same genetic material; these twins are known as monozygotic.