15.5 - Formation of Twins

Formation of Identical and Fraternal Twins

Identical Twins

• One sperm fertilises one ovum to form one zygote.
• The embryo divides completely into two embryos.
• Both foetuses share one placenta.

Fraternal Twins

• Two sperms fertilise two ova to form two zygotes.
• Two separate embryos form; the embryos do not divide into two.
• Each foetus has its own placenta.

Comparison between Identical and Fraternal Twins

Identical twins	Fraternal twins
Form from one zygote.	Form from two different zygotes.
The embryo divides into two.	The embryos do not divide into two.
Share one placenta.	Each has its own placenta.
Genetic makeup and physical appearance are similar.	Genetic makeup and physical appearance are different.
Both are the same sex.	May be the same or different sexes.

Development of Conjoined Twins

• Conjoined twins form when the embryo of identical twins divides incompletely.
• Two foetuses form, but some physical parts remain joined, usually the chest, abdomen or buttocks.
• They may share one or two internal organs.
• Surgical separation depends on the joined parts and shared internal organs.