People are intrigued by twins, and thanks to the progress in fertility science, there are more twins now than any other moment in history. Following the Centers for Disease Control and Prevention (CDC), in 2017, 33 out of every 1,000 births in the US were twins.
Identical and fraternal twins are the most widespread, but there are also other unusual types. Kindly read further to know more about these twins.
Identical twins are also called monozygotic twins, which mean one fertilized egg. They emerge when one egg is fertilized by a sperm as usual, but the egg breaks in two after that. Each half then develops into a baby.
Because they initially came from a similar egg and sperm, 100 per cent of their chromosomes are the same. This implies that they are of the same sex and have similar genetic traits such as hair and eye colour.
Nonetheless, things in their surroundings like how much room they each occupy in the womb can cause tiny discrepancies in their appearance.
Fraternal twins are also called dizygotic twins, which mean two fertilized eggs. They are the result of a mother releasing two eggs at the same time, thereby having each egg fertilized by a different sperm.
Because they come from separate eggs and sperm, they only share around 50 per cent of their chromosomes just like any other siblings. This suggests that they can be of the same or different sexes and are not identical.
Is there a third type?
There might be a third type, which is referred to as polar body or half-identical twins. Some doctors suggest that this would clarify why some fraternal twins have a similar appearance, but it hasn’t been verified that this type exists.
When an egg is released, it can divide in two. The smaller part of the two halves is known as a polar body. It has all things it requires to develop into a baby if it is fertilized. Nonetheless, there is relatively small fluid (cytoplasm) inside it, so it is normally too little to survive.
If a polar body survives, it could be fertilized by one sperm when another fertilizes the larger half of the egg. The outcome would be polar twins. Being that they come from one egg, the chromosomes from their mother are similar. But they do not share chromosomes from their father, and they may or may not be of the same sex.
Unusual occurrences of twin pregnancies
The majority of twin pregnancies end with two strong babies being born. Sometimes, unusual occurrences occur during fertilization or at an early phase of a twin pregnancy that produces unique twins, which include:
Mirror image twins
This is a subtype of identical twins, which occurs when the egg divides between 7 – 12 days after fertilization, rather than the first week. As of now, the growing embryo has already developed the left and right side.
These twins are similar but mirror images of each other. For instance, their hair may curl in different directions, their teeth may begin to come in on opposite areas of their mouth, and one might be right-handed while the other is left-handed.
They could even cross their legs in different directions.
These are identical twins that are physically bound to each other. Few doctors assert that this occurs because of a fertilized egg not dividing completely. This can transpire when it splits 12 days or more after conception. Others say it is an egg that split up completely but later linked back together.
The area of fusion differs, but it is most generally the chest or abdomen. The degree of fusion also varies, but the twins share one or more significant organs. Conjoined twins usually die before or soon after they are born.
Those that survive can occasionally be separated based on where they are joined and the organs they share. However, these twins are two individuals who can reason independently.
Parasitic twins are a type of conjoined twin where the smaller twin depends on the bigger twin. The smaller twin isn’t completely formed and might not have vital organs like a fully developed brain or heart.
The little twin can be formed anywhere on the other twin’s body and emerge as anything such as a tiny unnoticeable lump, a second broken head, or additional limbs bound to random body parts.
Subtypes of parasitic twins include:
Fetus in fetu
This occurs when a parasitic twin grows inside the bigger twin’s body instead of the outer part of it.
This is a twin to twin transfusion syndrome, which happens when an identical twin receives too much blood flow, and the other receives too little via a shared placenta. Acardiac twins have a serious form of this issue when the smaller twin is only a body with or without legs whose heart is malformed or missing.
This kind is the aftermath of two individual sperms fertilizing one egg. For this egg to survive, it must divide into two with each half getting exactly the right amount of chromosomes. There are only two known cases of semi-identical twins.
Boy or girl identical (monozygotic) twins
In some cases, identical twins can have different sex. These twins start as identical male twins. Just like every male, they both have XY sex chromosomes, rather than XX like every female do.
Once the egg divides into two, a genetic transformation prompts one twin to forfeit its Y sex chromosome, altering it to X0. This mutation is referred to as Turner syndrome.
Since there is just one X chromosome, the twin appears like females, but it tends to have developmental problems, which begins at birth and fertility issues later in life. This condition doesn’t affect the other baby.
Unusual fraternal twins
They include the following:
Twins with different ages
Superfetation means the fertilization of a second egg in a pregnant woman. It is very unusual because women often cease releasing eggs after they get pregnant. When this occurs during the same menstrual cycle, it is referred to as superfecundation.
Twins with separate fathers
Heteropaternal superfecundation occurs when individual fathers fertilize the two eggs released at separate times in the same ovulation cycle. It is widespread in animals but very unusual in humans.
Twins of unique races
This can occur naturally in three ways, but all are extremely unlikely:
- Fraternal twins are born to parents who have different nationalities: One of the twins happens to retain all the mother’s characteristics while the other looks like the father.
- Heteropaternal superfecundation occurs when the two fathers are different races. Each twin has the characteristics of their father’s race.
- When both parents are biracial: The genes in the sperm or egg of a biracial individual often lead to features that are a combination of both races. Nonetheless, if the genes from the sperm as well as the egg for one twin lead particularly to the characteristics of a single race, the genes for the other twin lead mainly to the features of the other race. The twins will look different.
Medical dangers during a twin pregnancy
Pregnancies with multiple fetuses are always considered high risk due to the fact that they may have more possibility of complications that include:
- Placenta preview: The placenta is low and surrounds the cervix.
- Placental abruption: The placenta rips off from the uterine wall.
- Placenta accreta: Blood vessels and other aspects of the placenta develop too deeply into the uterine wall.
- Prematurity: A baby is born before the 37th week of pregnancy.
- Gestational hypertension: The mother has high blood pressure during pregnancy
- Low birth weight: Birth weight is smaller than 5 pounds, 8 ounces.
- Gestational diabetes: A mother has a high level of sugar in her blood during pregnancy.
- Postpartum hemorrhage: A woman has severe bleeding once the delivery is done. This is because the placenta is big, and the uterus has spread too much.
Most twins are identical or fraternal, and there may be a third type referred to as polar body twins. Unique twins may emerge when unusual situations arise around the moment of fertilization or early growth.