About 1 in 90 spontaneous human births results from a twin pregnancy.twins

There are 3 different kind of twins:

  1. Dizygotic twins
  2. Monozygotic twins (dichorionic or monochorionic)
  3. Polar body twins

Dizygotic are twins which result from the fertilization of two different eggs with two different sperms. Other words or dizygotic twins are also fraternal or non-identical twins.

The babies will be no more alike than siblings born at separate times.

The babies can be either the same sex or different sexes with the probability roughly equal for each.

The major factors present in the woman that may cause dizygotic twin pregnancies are:

  1. Use of fertility drugs
  2. You are over 40
  3. Had several previous pregnancies
  4. Family history of dizygotic twins.

All dizygotic twins have two separate placentas and are in different sacs.

Monozygotic twins result from the fertilization of one egg and one sperm. The fertilized embryo splits days after fertilization in two individuals which share (usually) the same chromosomes.

twins 2

Polar body twins happen very rarely and they result from one egg fertilized by two different sperm.

Even that the girls in the picture do not resemble to one another they are TWINS!!

Lucy (the girl on the left) has fair skin, blue eyes and red hair, on the other hand Maria, has darker skin and brown eyes. The girl’s mother is half Jamaican which means that she carries genes for black and white skin. Randomly Lucy inherited the genes for white skin and Maria the genes for black skin.

Infertility treatments increase the rate of dizygotic twins, with about 35% of pregnancies from IVF (in vitro fertilization) being twin pregnancies.



  1. http://www.iflscience.com/health-and-medicine/interesting-genetics-these-girls-are-twins
  2. http://bestoftwins.com/wp-content/uploads/2012/07/monozygotic-dizygotic-twins.jpg (figure 1)
  3. http://www.babymed.com/articles/twins-monozygotic-vs-dizygotic-and-monochorionic-vs-dichorionic.

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