In addition to all the different combinations of identical and fraternal multiples, there are some rare types of twins.
Mirror-image twins occur only in identical twins. In approximately 23 percent of identical twins the egg splits later than usual, most often day seven or beyond. The original right half of the egg becomes one individual and the original left half becomes the other. These twins will often have "mirror images" of their features, such as hair whorls that run clockwise in one and counter clockwise in the other, a birthmark on the right shoulder of one and the left shoulder of the other, etc. There is no specific test for determining if twins are mirror-image. The determination is made by observation only, and the twins must be monozygotic, or identical.
One twin will be right-handed, while the co-twin is left-handed. This may be a partial explanation for the fact that a little over one third of identical twins are left-handed, double the rate in the general population. In extreme cases, all of the internal organs are reversed in one of the twins, with the heart on the right, the liver on the left and the appendix on the left.
Polar Body Twins (Half Identical)
Polar body twinning is very unusual and very rare. The process is quite complicated. The polar body appears when the egg has been developing, even before fertilization. It is a small cell that does not function and will usually degenerate and die. It is thought that in some cases, when the egg is old, the splitting off of the polar body takes place in an abnormal way. It then becomes larger, receives more nourishment, and does not die as it usually does. Instead, it acts as a second egg. The polar body and the egg share identical genes from the mother, but they may then be fertilized by two separate sperm from the father. This will result in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). They share some features of identical twins and some features of fraternal twins and thus are called half-identical twins.
Mixed Chromosomes or Chimerism
Another form of twinning that has been identified is called chimerism. This is thought to occur if two separate sperm fertilize two separate eggs which then fuse, producing individuals with different sets of chromosomes. Some have been identified that have more than one distinct red blood cell type and individuals who are both XX and XY (the sex chromosomes - XX being female and XY being male.) This phenomenon might also be associated with fused placentas causing intermixing of the circulations. It is very rare, and fewer than twenty-five cases have been identified. It is more common in other mammals, such as calves.
Twins can have different fathers. One well-known case was described in 1810 in the United States. A woman had both a white and a black lover, and she became pregnant and gave birth to twins, one white and the other black. Each twin had a different father. This is called superfecundation. It happens when the mother ovulates more than one egg and has more than one partner during her fertile period. One egg is fertilized with sperm from one partner, and the other egg from sperm of the second partner. These types of twins are always fraternal or dizygotic.
Superfetation occurs when a women ovulates more than one egg but the eggs are released at different times, sometimes up to 24 days apart, and they are fertilized when they are released. The resulting twin pregnancy has different conception dates, so the babies may be quite different in size. Days or weeks may separate the births. It is quite an unusual event. In some cases, the births of twins may be weeks or months apart due to deliberate medical intervention. This is called interval birth.