A twin is one of two offspring produced in the same pregnancy. Twins can either be Monozygotic or Dizygotic.
- 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 sperm.
In contrast, a fetus which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth is multiple. It is theoretically possible for two singletons to be identical if all 23 chromosomes in both gametes from the mother
and father were to be exact matches from one birth to the next. While this is statistically improbable (less than one in one billion-billion-billion chance) under natural conditions, a controlled pairing may someday be possible.
Multiple pregnancies are on the rise in recent years with more and more
twins and other types of multiples being born. The majority of the
reason for this rise is the increased use of fertility drugs.
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