There may be more twins in the United States than you might think. As of 2018, the Centers for

Disease Control and Prevention (CDC) reported that 32.6 out of every 1,000

Trusted Source births in the United States were twins.

You may be familiar with the most common types of twins — fraternal and identical — but several other rare subtypes exist, too. Read on for more details about twins.

Monozygotic, or identical, twins begin with a single egg. With this type of twin pair, one egg is fertilized by one sperm as usual, but then it splits into two eggs shortly after that. Each egg half then develops into a baby.

The chromosomes in both babies are 100 percent identical because both come from the same egg and sperm. For this reason, both children are assigned the same sex at birth and share the same genetic characteristics, such as eye and hair color.

Still, due to different experiences in their birthing environment (like the amount of space each had in utero), they may have slight differences in appearance.

Not all twins are identical, of course. More often than not, twins are born with unique physical characteristics. Nonidentical twins are generally known as fraternal twins.

Fraternal twins

The scientific term for fraternal twins — “dizygotic” — refers to two fertilized eggs. Dizygotic twins occur when the birthing parent’s body releases two eggs at the same time. A different sperm will fertilize each egg.

Since fraternal twins are the result of different eggs and different sperm, they share the same percentage of chromosomes as any other siblings, about 50 percent. This is why they don’t look exactly alike and can be different assigned different sexes at birth.

Traditionally, the science around twins has taught that identical and fraternal are the only two types. But there may be a third type, called polar body or half-identical twins.

Though this has never been confirmed definitively, some researchers have suggested that a third twin type would explain why some fraternal twins look so similar.

After an egg is released, it can split into two halves — the smaller of which is called a polar body. This polar body contains all the chromosomes necessary to join with a sperm to create a baby. But since it usually contains very little fluid, or cytoplasm, it’s often too small to survive.

It’s possible, though, that a polar body could survive and be fertilized. (Meanwhile, the larger half of the original egg could also be fertilized by a separate sperm.) The result? Polar twins.

Polar twins share the same chromosomes from their birthing parent but receive different chromosomes from the non-birthing parent. This is because they’re created from a single egg but two separate sperm.

For this reason, they may or may not be assigned the same sex at birth, and they may look very similar but not exactly identical.

When all goes smoothly, a twin pregnancy results in two healthy babies, whether identical or fraternal. But some events in utero can lead to unique twins.

Mirror twins

Mirror twins are exactly what they sound like! These twins are actually mirrored images of each other. This means that their hair may naturally fall in opposite directions, their teeth may grow in on opposite sides of their mouth, and they may have birthmarks on the opposite side of their bodies. They also usually have different dominant hands.

What causes this phenomenon? In a typical twin pregnancy, an egg splits during its first week after fertilization. But in mirror twins, the egg splits 7 to 12 days after it’s been fertilized — long enough for it to have developed a right and a left side.

Conjoined twins

Conjoined twins are a rare twin type in which the two siblings are physically connected. Typically, conjoined twins are fused together at the chest or abdomen, but the point of fusion varies. Some conjoined twins are connected to a greater extent than others, but most share at least one vital organ.

Though physically attached to each other, conjoined twins are two individuals with unique thoughts and personalities.

There is still some mystery around the origin of this type of twin birth. Some experts believe conjoined twins occur when a fertilized egg doesn’t split completely when it divides 12 or more days after conception. Another theory is that the fertilized egg divides completely but later fuses back together.

Many pairs of conjoined twins don’t survive long after birth. When they do survive, it’s sometimes possible for them to be surgically separated, depending on the extent of their connection and the organs shared.

Parasitic twins

Sometimes, as twins develop in utero, one becomes larger and more dominant, while the other ceases to develop and depends on its sibling. Known as parasitic twins, these twins are physically conjoined.

However, the smaller twin is not fully formed and isn’t capable of surviving on their own. This is because the smaller twin is usually missing vital organs or lacking a fully developed brain or heart.

In fact, the smaller twin is often not recognizable as a separate individual. This twin may appear on the sibling’s body as a small lump, extra limbs, or a second nonfunctioning head.

Parasitic twins may be classified into subtypes, including fetus in fetu and acardiac twins.

Fetus in fetu occurs in rare circumstances when a smaller twin develops inside the larger twin’s body.

Acardiac twins happens when one twin receives too much blood flow while the other doesn’t receive enough. This occurs because identical twins share a placenta.

A milder version of this is called twin to twin transfusion syndrome. Acardiac twins experience a more extreme form of this phenomenon that may result in a missing or malformed heart or only a torso that may or may not have legs, according to 2015 researchTrusted Source.

Semi-identical twins

There have only been two reported cases of semi-identical twins, so this type is extremely rare.

In semi-identical twins, two separate sperm fertilize one egg. The fertilized egg then splits in two. So, semi-identical twins share all the same chromosomes from their birthing parent, but only about 50 percent from their non-birthing parent.

Boy/girl monozygotic (identical) twins

Sometimes identical twins can be assigned the sex of a boy and a girl at birth. These twins start off as identical males with XY sex chromosomes. But shortly after the egg divides, a genetic mutation called Turner syndrome occurs, leaving one twin with the chromosomes X0.

This twin will look female but may have developmental concerns from birth, as well as difficulties with fertility later in life, according to the National Health Service. The mutation doesn’t affect the other twin.

Twins with different ages

Once a person becomes pregnant, their body stops releasing new eggs for potential fertilization — except in some rare cases.

A phenomenon known as superfetation can occur when a second egg gets released and fertilized after a person is already pregnant. When this happens twice within one menstrual cycle, it’s known as superfecundation.

In this case, both fertilized eggs will develop, but one twin will be slightly older than the other.

Twins with different fathers

If two eggs are released within a single menstrual cycle, it’s possible for them to be fertilized by two different non-birthing people.

This is known as heteropaternal superfecundation — a common occurrence in animals but very rare in people.

Twins of different races

Twins of different races are very unlikely but can occur in three different ways:

  • When parents are of different races, one of their fraternal twins may naturally resemble the birthing parent, while the other looks like the non-birthing parent.
  • In heteropaternal superfecundation, the two non-birthing partners may be from different races. Each twin would then receive that person’s racial genetics.
  • When both parents are biracial, this usually results in twins that both look biracial. Occasionally, though, each twin may receive more genetic material from one race than the other. This can lead to the twins appearing to be from different races.

Twin pregnancies often come with increased risk due to a higher likelihood of several medical conditions. These can include:

  • Placenta previa. In placenta previa, the placenta sits too low in the uterus, covering the cervix.
  • Placental abruption. With placental abruption, the placenta detaches from the uterine wall, causing bleeding, back pain, and abdominal tenderness.
  • Placenta accreta. The placenta attaches too deeply into the uterine wall when placenta accreta occurs.
  • Prematurity. A premature baby is one who is born too early (before 37 weeks of pregnancy).
  • Low birth weight. A baby with low birth weight is born with a weight that’s less than 5 pounds, 8 ounces.
  • Gestational diabetes. In gestational diabetes, the birthing parent’s blood sugar is too high during pregnancy.
  • Gestational hypertension. In gestational hypertension, the birthing parent’s blood pressure is too high during pregnancy.
  • Postpartum hemorrhage. When a postpartum hemorrhage occurs, the birthing parent experiences heavy bleeding after delivery, often because of an undelivered placenta or the uterus not contracting properly.

Most twins are fraternal or identical, but a third type — polar body twins — might exist, too.

Some other subtypes of unique twins can occur under unexpected developmental circumstances in the womb, but these are extremely rare.