There are two main types of twins – Identical twins (or monozygotic twins) and Fraternal twins (or dizygotic twins). But there are rare and unusual types of twins that exist beyond the standard Identical and Fraternal twins that we all know.
In most cases, women give birth to one child at a time but in certain cases, women give birth to multiple children called twins or triplets. According to the Centers for Disease Control and Prevention (CDC), 33 out of every 1,000 births in the United States were twins.
Read more to find out the difference between common and rare types of twins. You may not be heard about these unique and unusual types of twinning before.
10 Unique Types of Twins:
Identical twins, also called monozygotic twins develop from a single fertilized egg, which then divides making two identical babies. Identical twins are genetic doppelgangers because they originally came from the same egg and the same sperm and 100% of their chromosomes are identical so for this reason they are showing the same sex, eye color, and hair color.
The slight difference in their appearance is caused by changes in the environment such as how much room they each had in the womb etc.
2. Fraternal Twins
Fraternal twins, also called dizygotic twins mean two fertilized eggs. They are formed from the fertilization of two eggs by two different sperm in the womb.
With fraternal twins, the two fetuses (developing babies) each have a separate placenta, inner membrane and outer membrane. They don’t usually look identical and might or might not be the same sex.
Fraternal twins are more common than identical twins. They are just two regular siblings that happen to be born at the same time.
Most parents are familiar with two common types of twins – Identical and Fraternal. Apart from these two main types of twins, there are some rare and unique types of twinning. Let’s discuss it in detail.
3. Conjoined Twins
Conjoined twins are identical twins that are physically connected to each other and do not fully separate from each other. Some doctors say it is due to incompletely division of fertilized egg. Others say it’s an egg that divided completely but later fused back together.
They are connected to certain parts of the body and may share tissue, limbs, or organs. Conjoined twins are often die or before birth or shortly after they are born.
4. Parasitic Twins
Parasitic twins also known as an asymmetrical or unequal conjoined twin are a type of conjoined twins, in this case, the smaller twin is leaning on a larger or complete twin. The smaller one is not fully developed and may not have important body organs such as a fully developed brain, head, or heart.
The smaller twin can be formed anywhere on the larger twin body and appears as any additional organ such as a small limb, a second nonfunctional head, or other extra body parts.
The undeveloped twin is called parasitic and the larger twin is called the autosite.
5. Semi-Identical Twins
Semi-identical twins also known as “sesquizygotic” are formed when two separate sperm fertilizing in a single zygote. For survival, the egg must be split into two and each half should have the exact number of chromosomes.
As of 2020, only two cases have been reported of semi-identical twins.
6. Mirror-Image Twins
Mirror-image twins are a type of identical twins which formed from a single fertilized egg which split later than the normal timing around 9 to 12 days. As of the term mirror-image, this type of twinning can develop reverse asymmetric features or opposite to each other. For example, if one individual is right-handed the other will be left-handed, have birthmarks on the opposite side of the body, or having hair whorl that swirls on the opposite side. When the twin faces each other, they would appear as an exact reflection of each other. In stats, about 25% of identical twins are mirror-image twins.
7. Boy/Girl Identical Twins
Identical twins are always same-sex because they formed from a single zygote that contains either male XY or female XX sex chromosomes, but in extremely rare cases identical twins have been born with different sexes i-e one male and one female.
This is because if the egg is fertilized by a male sperm XY and during cell division the one twin misses Y chromosomes. This results in one normal male XY and one female XX. The female may have Turner’s syndrome characterized by the short and webbed neck, short stature, and swollen hands and feet.
8. Twins with different ages: Superfetation
The fertilization of a second egg in the woman who is already pregnant is called Superfetation. It is very rare because women usually stop releasing eggs as they are getting pregnant. Normally, when an egg is fertilized the women’s ovulation is ceased. The second egg results in twin’s birth at different times.
9. Twins with different fathers: Heteropaternal superfecundation
When two eggs are released at a different time in the same ovulation period by different partners is called Heteropaternal superfecundation. However, it is common in animals and rare in humans.
10. Twins of Different races
In Heteropaternal superfecundation when the twins have different racial characteristics. It is due to three reasons.
- Twins that are born to parents who are different races. One individual has all mother’s race features while the others have father characteristics.
- In the Heteropaternal superfecundation case where the two fathers are different races. Each twin has their own father’s race characteristics.
- When both parent are biracial, the gene in the sperm or egg of the biracial usually has a mixture of races. So, if the genes from both sperm and egg for one twin usually lead to one race while the genes for the twin lead mostly to the other race. The twins look like different races.