A twin is one of two offspring produced in the same pregnancy and have shared the same uterus.
Twins can either be monozygotic (identical), meaning that they develop from one zygote that splits and forms two embryos, or dizygotic (fraternal) because they develop from two separate eggs that are fertilized by two separate sperms.
Fraternal Twins (non-identical twins) usually occur when two fertilised eggs are implanted in the uterine wall at the same time, i.e. when the mother releases two eggs and both become fertilized by two different sperms. The two eggs form two zygotes and these twins are therefore also known as dizygotic.
Dizygotic twins, like any siblings, have a number of different chromosones that distinguish them. Dizygotic twins may be a different sex or the same sex.
Identical Twins occur when a single egg is fertilized by a single sperm to form one zygote (monozygotic) but the zygote then divides into two separate embryos. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic).