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Twins are two offspring produced by the same pregnancy. Twins can be either monozygotic ('identical'), meaning that they develop from one zygote, which splits and forms two embryos, or dizygotic ('non-identical' or 'fraternal'), meaning that each twin develops from a separate egg and each egg is fertilized by its own sperm cell. Since identical twins develop from one zygote, they will share the same sex, while fraternal twins may or may not. In very rare cases twins can have the same mother and different fathers (heteropaternal superfecundation).

In contrast, a fetus that develops alone in the womb (the much more common case in humans) is called a singleton, and the general term for one offspring of a multiple birth is a multiple. Unrelated look-alikes whose resemblance parallels that of twins are referred to as doppelgänger.