15.5 - Formation of Twins

Process of Twin Formation

Occasionally, when two or more secondary oocytes are being released and fertilized simultaneously, two or more embryos will be formed. If two embryos are formed, they are called twins. Twins are either fraternal or identical.

Figure 10 - Identical twins (left) share the same placenta. Fraternal twins (right) have separate placentae.

Comparison between Identical twins and Fraternal twins

• Similarities

  • Both embryos are formed as a result of fertilization between an ovum and a sperm
  • Both grow by mitosis

• Differences

  • Identical twins

    • Fertilization: Involves one ovum and one sperm to form one zygote
    • Number of placentae: One placenta shared between two fetuses
    • Sex: Same sex
    • Genetic constitution and physical characteristics: Similar
    • Division of the embryo: Embryo divides into two

  • Fraternal twins

    • Fertilization: Involves two sperms and two ova to form two zygotes
    • Number of placentae: Each fetus has own placenta
    • Sex: May be same or different
    • Genetic constitution and physical characteristics: Different
    • Division of the embryo: No division

Development of conjoined twins

Incomplete division of the embryos will result in the development of conjoined twins. Their physical parts are attached together, such as the chest, abdomen or buttocks. Some of them may share one or two internal organs. Conjoined twins may face movement limitations as they are bound together. Most of them die before birth or early before they reach adulthood. They can be separated through surgical procedures, but the chances of success depend very much on their joined parts or shared internal organs.