Most twins are fraternal or identical, but there are additional subtypes, including mirror twins and conjoined twins, that are rare.

You may be familiar with the most common types of twins — fraternal and identical — but several other rare subtypes exist too.

More twins are born than you might think. As of 2019, the Centers for Disease Control and Prevention (CDC) reported that 32.1 out of every 1,000 births in the United States were twins. Read on for more details about twins.

The National Library of Medicine says that monozygotic, or identical, twins are conceived from one fertilized egg. This egg separates into two embryos after it has begun to divide. These two embryos develop into two babies.

Genetic materials called chromosomes in both babies are completely identical. This is because both babies 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, because of differences in the environment where they’re born (like the amount of space each had in the uterus), identical twins 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 happen 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. The National Human Genome Research Institute says that this is about 50 percent. This is why they don’t look exactly alike and can be assigned different sexes at birth.

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

Though this has never been confirmed, a research review from 2016 suggested that a third twin type would explain why some fraternal twins look so similar.

After the ovaries release an egg, the egg 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 they get different chromosomes from their 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.

In a basic twin pregnancy, two embryos go their separate ways and develop into twin babies, whether identical or fraternal. Some unique sets of twins follow a different path.

Mirror twins

Mirror twins are exactly what they sound like! These twins are actual mirror 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 mouths.
  • 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 identical twin pregnancy, an egg splits during its first week after fertilization. But in a mirror twin pregnancy, the egg splits 7 to 12 days after it’s been fertilized — long enough for the egg 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 joined together at the chest or abdomen, but this varies. Some conjoined twins are connected to a greater extent than others. Most share at least one vital organ.

Though physically attached to each other, conjoined twins are two individuals. They have unique thoughts and their own personalities.

Researchers are still trying to understand the origins of this type of twin birth. Some experts believe conjoined twins occur when a fertilized egg doesn’t split completely. This happens when the egg divides 12 or more days after conception. Another theory is that the fertilized egg divides completely, but it later fuses back together.

A conjoined twin pregnancy is high-risk, but breakthroughs in surgery have been able to improve outcomes. Surgery can even allow conjoined twins to live independently. Although a small 2011 study found that just 7.5 percent of conjoined twins live past birth, advances in care for conjoined twins are being made every year.

Parasitic twins

Sometimes, as twins develop in utero, one twin becomes larger and more dominant. The other twin stops developing and begins to depend on their 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 a fully developed brain or heart.

In fact, you might not even recognize the smaller twin as a separate individual. This “twin” may appear on the sibling’s body as a small lump, extra limbs, or a second non-functioning head.

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

  • Fetus in fetu. These are rare circumstances when a smaller twin develops inside the larger twin’s body.
  • Acardiac twins. In this case, 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 (TTTS). Acardiac twins experience a more extreme form of TTTS that may result in developmental issues for the fetuses. Treatments like surgery can improve outcomes, according to 2015 research.

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.

Female and male identical twins

Sometimes identical twins can be assigned the sex of male and female 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 be assigned female at birth but may have developmental issues, as well as difficulties with fertility later in life, according to the National Health Service. The mutation doesn’t affect the other twin, who is assigned male at birth.

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 sperm from two different people.

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

Twins with different skin colors

It’s very unlikely to have twins with different skin tones, but it can happen in three different ways:

  • When parents are of different complexions, one of their fraternal twins may naturally resemble one parent, while the other twin looks more like the other parent.
  • In the rare case of heteropaternal superfecundation, the two non-birthing parents may have different skin tones. Each twin would then receive that person’s 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 different ancestors than the other. This can lead to the twins appearing to be from different “races.”

Twin pregnancies often come with an increased risk of developing some 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 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 and 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 exist, but these are extremely rare. Many occur under unusual circumstances.

A twin pregnancy has risks of complications, so parents should make sure to receive good prenatal care and seek medical attention for any concerns.