Twins :- 

1. When a mother gives birth to two children at a time then these children are said to be twins . 

There are two types of twins : 

1. Monozygotic twins
2. Dizotic twins

Monozygotic twins :-

These are the twins which are formed from fertilization , and by formation of one zygote . THese are of same gender and have same genetics.

Types of monozygotic twins :- 

1. Monozygotic bichorionic :-  

• ABUNDANCY : 25% - 35%
• CELL STAGE : Two cell stage
• MECHANISM : The blostomere is divided into two zygotes , having two separate chorion so called bichorionic .

     2. Monozygotic biamnionic :- 

• ABUNDANCY : 70% - 75%
• CELL STAGE : Blastocyst
• MECHANISM : The inner cell mass present in the balstocyst divides into two having ; same placenta , same chorion and two different amnion so called bi amnionic . 

    3. Monochorionic Monoamnionic :- 

• ABUNDANCY : 1%- 2%
• CELL SATGE : In the bilaminar germ disc
• MECHANISM : Two separate organising centres appear and disc gives rise to two separate embroyos , they lie in single amnion and single chorion , so called monochorionic monoamnionic.

Dizotic twins :- 

• These are the twins formed by fertilization of two oocytes by two sperms .
• The 2/3rd twins are dizotic
• These may be of same gender or different 
• These individuals will have different genetics.

NOTE : 

• If the twins are not separated then they are called conjoint twins .
• If one of the twin is having diminished blood supply nutrients and etc... then, the twin will be called as parasitic twin .