Fraternal Twins (Produced from two eggs)
Some times two eggs are released from ovaries at a time and both may get fertilized to produce two individuals. Such twins produced from two eggs are called fraternal twins.
Fraternal twins may be either both boys, or both girl, or one boy and one girl.
Also known as Diovular or Dizygotic twins.
About 75% of twins belonging to this type of twins.
Identical twins (Produced from one egg)
Such twins are produced when a single fertilized egg may get split and separated in to two parts during its early stages of cell division & each pair behave like an independent egg and produces one complete individual each.
Identical twins being produced from a single egg are either both boys or both girls & very similar to each other.
Fingerprints, Birthmarks, Handwriting are some characteristics which are not similar even in identical twins.
When one single zygote split in to 2, and one of the split parts again splitting in to 2 then the triplets may be produced.
Quadrapulates may be produced as two pairs of identical twins from two eggs or in other combination of identical and fraternal twins.
Test Tube Babies (vitro fertilazation)
Some times, the mother are incapable of conceiving for some reasons then the mature ovum taken from their ovary and it is made to fertilize in a petri dish (in vitro fertilization, "vitro" means "glass" referring to fertilization in glass containers) and allowed to develop up to a certain stage.
There after, it is placed back in the uterus for further natural growth and the children born are called "test tube" babies.