<P> 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 ("fraternal"), meaning that they develop from two different eggs . In fraternal twins, each twin is fertilized by its own sperm cell . </P> <P> In contrast, a fetus that develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple . Non-related look - alikes whose resemblance parallels that of twins are referred to as doppelgangers . </P>

What are the different types of twin pregnancies