Twin

Mauch twins

Red-haired siblings

Fraternal twin brothers

Mirjam Weichselbraun and Melanie Binder

Diagram of identical and fraternal twins

Mark and Scott Kelly

Types of placentation

A twin is one of two offspring produced in the same pregnancy. Twins can be either monozygotic ("identical"), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ("fraternal"), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell.

Types of Twins

Monozygotic Twins

Monozygotic twins, also known as identical twins, occur when a single fertilized egg splits and develops into two embryos. These twins share the same genetic material and are always of the same sex. The splitting of the zygote can occur at various stages, leading to different types of placentation, such as monochorionic or dichorionic.

Dizygotic Twins

Dizygotic twins, or fraternal twins, result from the fertilization of two separate eggs by two separate sperm cells. These twins are genetically similar to regular siblings, sharing about 50% of their DNA. They can be of the same or different sexes and have separate placentas.

Causes of Twinning

The occurrence of twins can be influenced by several factors, including genetics, maternal age, and fertility treatments. Women who have a family history of twins, particularly on the maternal side, are more likely to have twins themselves. Older mothers and those undergoing fertility treatments are also at a higher risk of having twins.

Complications

Twins are at a higher risk for certain complications during pregnancy and birth. These include twin-to-twin transfusion syndrome, preterm birth, and low birth weight. Monozygotic twins can also face complications related to shared placental structures, such as twin reversed arterial perfusion (TRAP) sequence.

Cultural Significance

Twins have held a special place in various cultures throughout history. In some cultures, twins are considered lucky or are believed to possess special powers. In others, they may be viewed with suspicion or fear. The Yoruba people of Nigeria, for example, have one of the highest rates of twinning in the world and have a rich cultural tradition surrounding twins.

Notable Twins

Some famous twins include the Bunker brothers, Chang and Eng, who were conjoined twins from Siam (now Thailand) and became famous in the 19th century. In modern times, Mark and Scott Kelly are notable identical twins who have both served as astronauts.

Gallery

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  Dichorionic twins

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  Monoamniotic twins

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  Yoruba twin figures

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  Chang and Eng Bunker

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  Twin-to-twin transfusion syndrome

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  Hereford twins

Related Pages

• Multiple birth
• Conjoined twins
• Genetics
• Fertility treatment