The term “twin” refers to two individuals who have shared the same womb, and usually, but not necessarily, share the same birthday. Twinning normally occurs in two ways: – either the woman releases only one ovum (egg) which divides/splits after fertilization, or she produces two separate eggs rather than the usual one. However, the type of twin that is eventually conceived depends on what happens to the egg(s) during development. This article defines and describes the top ten unusual and unique types of twins.
10. Conjoined Twins
Conjoined twins are twins which are genetically identical. They are always of the same sex and cannot be separate sexes. Besides developing from one fertilized egg, they also share same placenta and amniotic cavity. These twins take intimacy to the extreme; sharing legs, arms, hearts, and other body organs – even brains. Conjoined twins develop when a woman produces only one single egg which fails to fully split after fertilization. Normally, the single embryo begins to split a few weeks post-conception, but the separation stops it is complete. One in every 200,000 births are conjoined twins.
9. Half-Identical Twins
Half-identical or half-twins (or semi-identical twins and polar body twins) occur when the twins inherit similar genes from the mother, but totally different genes from the father. Theoretically, they are caused when the polar body is fertilized by only one sperm and at the same time, the egg is fertilized by a different sperm resulting in twins who share half their genes in common (from the mother). But, the other half of their genes are different since they come from two distinctively different sperm cells.
8. Twins of Different Race
Heteropaternal superfecundation can explain cases where dizygotic (fraternal) twins exhibit different racial characteristics, due to the different races of the parents. One child can be white, fair-haired, and light skin while the other can be black and dark-haired. Statistics show that the chance of a mixed race couple having twins of different colors is a one in a million.
7. Monozygotic Twins
Monozygotic twins (also called identical twins, enzygotic twins, true twins, or uni-ovular twins) are two offspring born of the same pregnancy, but developed from one zygote (a single fertilized ovum), which later split into two equal halves during the early phase of embryo development. This leads to the formation of two embryos, each of which develops as a separate but identical fetus. Monozygotic twins are always of same sex. They also have practically identical genes, possess identical blood groups, have the same genetic constitution, and closely resemble each other in their physical, mental, and psychological characteristics.
6. Dizygotic Twins (Fraternal Twins)
The term dizygotic means two fertilized eggs. Dizygotic twins, or fraternal twins, form when two eggs are fused and fertilized by two separate, resulting into two separate zygotes. In most circumstances, women only release a single egg in one ovulation cycle. However, sometimes multiple eggs may be released in one cycle, leading to dizygotic twins. They can be of the same sex or different sex. These twins have their own separate placentas and amniotic sac.
5. Mirror Image Twins
Mirror twins originate from one fertilized egg. They basically form the same way as monozygotic twins, but the only difference is that image twins tend to develop reverse asymmetric features. An example is when one is right handed, the other is left-handed. When they face each other, they are the exact mirror reflection of themselves. This is something that is normally attributed to a delayed split of the fertilized egg, often seven days after conception. Other reverse asymmetric features include opposite thumbs, hair whorls that swirl in opposite directions, and birthmarks on opposite sides of their body. It is said that 25% of identical twins (monozygotic twins) tend to be mirror image twins.
4. Twins with Different Birthdays
This is a situation where twin A is born at say, 11:56pm and the twin B is born at 12:02am the following day. Or, it could be in different years. Like twin number one is born on December 31st and the second one is born on January 1st, technically being the next year. It is also possible that if a baby is premature, the second twin will not be born until the day after.
3. Parasitic Twins
Parasitic twins are formed when conjoined twins develop asymmetrically/disproportionately with the stronger baby supporting the weaker one. The name comes from the fact that the weaker twin (parasitic twin) depends on the stronger one for survival. A parasitic twin may consist of abnormal cell growth, an example being extra limbs. The stronger twin may also develop an abnormal mass of cell inside its body, what is referred to as “fetus in fetu.”
2. Superfetation Twins
The term superfetation is used when a fetus is formed while another fetus is already in the uterus. Superfetation happens when a women ovulates more than one egg, but the eggs are released at different times. It can sometimes be up to twenty four days apart, and they are fertilized when they are released. An example would be when two separate occurrences of fertilization taking place during the same cycle. It is an unusual event among humans, but is commonly experienced among animals.
1. Heteropaternal Superfecundation Twins: Twins with Different Fathers
Twins can have different fathers. One well-known case was described in 1810 when an American 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 and 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.