Why are twins born in humans?

Usually, only one child is produced at a time in mammalian species including humans.

However, sometimes two or more children are born at the same time. Those we call twins. Why are twins born in humans? What are the ways? It is not uncommon for human mothers to give birth to more than one child at the same time.

Facts about identical twins

There are two kinds of twins,
  1. Fraternal twins or heterozygous twins
  2. Identical or homozygous twins

how are identical twins formed

Heterozygous twins- Usually, ovaries release only one egg at a time. It is fertilized and developed in the child. 

but sometimes ovaries release two eggs, one from each ovary. Both of these eggs fertilized and developed.

Such fraternal twins or asexual twins may be of the same or different sex and have the same degree of common brothers and sisters.

They are completely independent individuals who have about 25% of the genetic information in common, as do common siblings.

Although they may be located close together in the uterus, the fetal membranes of fraternal twins are separated.

Identical twins

True or identical twins, also termed homozygous twins, are the products of a single egg fertilized by a single sperm.

At some early stage of development, the egg divides into two (or more) independent parts, with each developing into a separate fetus.

Since they come from the same fertilized egg, they are genetically identical and, therefore are of the same sex. Such twins usually arise from the same blastocyst.

Two sepa­rate inner cell masses may arise from a single blastocyst or a single cell mass may divide into two. In these cases, the twins will have separate amnions and umbilical cords but share the chorion and placenta.

If, however, a single cell mass develops into two embryos, the twins will share amnion, chorion, and placenta.

Occasionally, identical twins develop without separating completely and are born joined together, and are termed, Siamese twins.

All grades of the union have been known to occur, from almost complete separation to fusion throughout most of the body so that only the head or legs are double.

Sometimes the two twins are of different sizes and degrees of development. One might be quite normal, while the other might be an incompletely formed parasite of the first.

Such errors of development usually cause death during or shortly after birth.