There are 7 different kinds of twins, categorised as Identical, Fraternal, Half- Identical, Mirror Image Twins, Mixed Chromosomes Twins, Superfecundation and Superfetation. Normally, except the first two kinds, others are known to be quite rare to occur. All these twins, apart from similar faces have some varied characteristics.
- Fraternal Twins
Fraternal twins or commonly known as “non-identical twins”, usually occur when two fertilized eggs are implanted in the uterine wall at the same time that is when the mother releases two eggs and both become fertilised by two different sperms. The two eggs form two zygotes and these twins are therefore called dizygotic.
Twins that are male and female or have different blood types can easily be determined as fraternal. They have very small chance of having the exact same chromosome profile. Fraternal twins can share many similarities in appearance just like any other brother or sister of the same family.
- Identical Twins
Identical twins occur when a single egg is fertilized by a single sperm to form one zygote or monozygotic. But the zygote then divides into two separate embryos. The two embryos develop into foetuses sharing the same womb.
Identical twins start out as genetically identical as they have the same genes. Later on, they develop their own individual personalities to enable themselves to be identified as individual persons. They are always of the same sex.
- Monoamniotic Twins
Monoamniotic twins share the same placenta, chorionic and amniotic sac. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta. This leads to twin-to-twin transfusion syndrome. About 50% of monoamniotic twins die from umbilical cord entanglement.
These twins generally look alike although examination of details such as fingerprints and teeth marks can tell them apart.
- Mirror Twins
Mirror twins also called Mirror Image Twins are a subset of identical twins and are identical with opposite features. That is one will be right handed and the other will be left handed. They result from a late split of the fertilized egg at around 9-12 days.
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.
Due to the limited size of the mother’s womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 37 weeks). Since some premature births often have health consequence to the babies, twin births are often handled with special procedures than regular births.