Twins are two offspring produced by the same pregnancy. They can either be identical (from one egg) or fraternal (from two separate eggs).

Manny mammals give birth to more than one offspring at a time.

Human twins may come from one zygote or two zygotes. The one that come from one zygote are identical and known as monozygotic twins - they have the same sex. Mono means one. About 33% of identical twins have separate placentas, but the other identical twins share a common placenta. Whereas from two zygotes are non-identical and they are known as dizygotic, di means two or fraternal twins.

Fraternal twins do not resemble each other. Always they have 2 placenta. They may be the same or different sex.