People are fascinated by twins, and thanks in large part to advances in fertility science, there are more twins than almost any other time in history. In fact, according to the Centers for Disease Control and Prevention (CDC), in 2017, 33 out of every 1,000 births in the United States were twins.

Identical and fraternal twins are the most common, but there are also several other rare types. Read on to learn more about twins.

Identical twins are also called monozygotic twins, meaning one fertilized egg. They occur when one egg is fertilized by one sperm as usual, but the egg splits in two shortly afterward. Each half then grows into a baby.

Because they originally came from the same egg and sperm, 100 percent of their chromosomes are identical. This means they’re the same sex and have the same genetic characteristics, like hair and eye color.

However, things in their environment, such as how much room they each had in the womb, can cause slight differences in their appearance.

The other name for fraternal twins is dizygotic twins, meaning two fertilized eggs. They’re the result of the mother releasing two eggs at the same time with each egg being fertilized by a different sperm.

Because they come from different eggs and sperm, they only share about 50 percent of their chromosomes like any other siblings. This means they can be the same or different sexes and aren’t identical.

There may be a third type called polar body or half-identical twins. Some doctors suggest this would explain why some fraternal twins look identical, but it’s never been proven that this type exists.

When an egg is released, it can split in two. The smaller of the two halves is called a polar body. It has everything it needs to grow into a baby if it’s fertilized. However, there’s very little fluid (cytoplasm) inside it, so it’s usually too small to survive.

If a polar body survives, it could be fertilized by one sperm when the larger half of the egg is fertilized by another. The result would be polar twins.

Because they come from a single egg, the chromosomes from their mother are identical. They share no chromosomes from their father. They may or may not be the same sex.

Most twin pregnancies end with two healthy babies being born. Occasionally, unusual events happen during fertilization or at an early stage of a twin pregnancy that leads to unique twins.

Mirror image twins

This is a subtype of identical twins that happens when the egg splits 7 to 12 days after fertilization, instead of during the first week. By this time, the growing embryo has already developed a left and right side.

These twins are identical but mirror images of each other.

For example, their hair may spiral in opposite directions, their teeth may start coming in on opposite sides of their mouth, and one may be right-handed while the other is left-handed. They may even cross their legs in opposite directions.

Conjoined twins

These are identical twins that are physically connected to each other.

Some doctors say it’s due to a fertilized egg not splitting completely. This can happen when it divides 12 days or more after conception. Others say it’s an egg that divided completely but later fused back together.

The location of fusion varies, but it’s most commonly the chest or abdomen. The extent of fusion also varies, but almost always the twins share one or more vital organs.

Conjoined twins often die before of shortly after they’re born. Those that survive can sometimes be separated depending on where they’re joined and which organs they share.

Although joined, these twins are two individual people who can think independently.

Parasitic twins

Parasitic twins are a kind of conjoined twin where a smaller twin is dependent on a larger one. The smaller twin isn’t fully formed and may not have vital organs such as a fully formed brain or heart.

The small twin can form anywhere on the other twin’s body and appear as anything such as a small unnoticeable lump, a second nonfunctional head, or extra limbs attached to random body parts.

Subtypes of parasitic twins include:

  • Fetus in fetu. This is when a parasitic twin develops inside the larger twin’s body rather than outside of it.
  • Acardiac twins. Twin to twin transfusion syndrome occurs when one identical twin gets too much blood flow and the other gets too little through a shared placenta. Acardiac twins have a severe form of this where the smaller twin is only a torso with or without legs whose heart is missing or malformed.

Semi-identical twins

This type is the result of two separate sperms fertilizing a single egg. To survive, this egg must then split into two with each half having exactly the right number of chromosomes.

There have only been two reported cases of semi-identical twins.

Boy/girl monozygotic (identical) twins

In very rare cases, identical twins can be different sexes. These twins begin as identical male twins. Like all males, they both have XY sex chromosomes, instead of XX like all females do.

Very soon after the egg splits in two, a genetic mutation causes one twin to lose its Y sex chromosome, changing it to X0. This mutation is called Turner syndrome.

Since there’s only one X chromosome, the twin looks female but has developmental difficulties starting at birth and fertility problems later in life. The other baby isn’t affected.

Twins with different ages

Superfetation refers to the fertilization of a second egg in a woman who’s already pregnant.

It’s very rare because women usually stop releasing eggs as soon as they get pregnant. When this happens during the same menstrual cycle it’s called superfecundation.

Twins with different fathers

Heteropaternal superfecundation is when two eggs released at different times in the same ovulation cycle are fertilized by different fathers. It’s common in animals but very rare in people.

Twins of different races

This can happen naturally in three ways, but all are very unlikely:

  • Fraternal twins are born to parents who are different races. One twin happens to have all the mother’s features while the other takes after the father.
  • Heteropaternal superfecundation where the two fathers are different races. Each twin has the features of their own father’s race.
  • Both parents are biracial. The genes in the sperm or egg of a biracial person usually lead to features that are a mixture of both races. However, if the genes from both the sperm and egg for one twin lead mostly to the features of one race while the genes for the other twin lead mostly to features of the other race, the twins will look like different races.

Pregnancies with multiple fetuses are often considered high risk because they may have more of a chance of difficulties such as:

Most twins are fraternal or identical. There may be a third type called polar body twins.

Unique twins may develop when unusual circumstances occur around the time of fertilization or early development.