Twins or twins are two or more individuals who divide the same uterus and are usually, but not always, born within the same day. In humans, mothers with wombs carrying twins will thus experience multiple labor and usually shorter periods of pregnancy (34 to 36 weeks) than a single infant. Since preterm delivery usually has health consequences for infants, twin births are often handled specifically rather differently than ordinary births.
Twinning is common in animals because many animals produce egg cells (ready to be fertilized) at the same time. Domestic animals such as cats, dogs, mice, or sheep usually have more than one child during childbirth. 
Typesoftwins
Differences of dizygotic (right) and monozygotic (left) twins. Judging from the origin of the zygote, two types of twinning are known: fraternal (dizygotic) and identical (monozygotic). Dizygotic twins are common in vertebrates, whereas monozygotic twins are rare. Man has this ability. Armadillo striped (Dacypus novemcinctus) if the birth always has monozygotic four twins.
Dizygotic or fraternal twins (DZ)
Dizygotic twins (known as "non-identical twins") occur because the zigots are formed from different eggs. There is more than one egg cell attached to the uterine wall that is fertilized by sperm cells at the same time. In humans, the process of ovulation sometimes releases more than one mature ovum to the fallopian tubes which, when they are fertilized, will produce more than one zygote. Dizygotic twins are genetically no different from ordinary siblings and develop in separate amnion and placenta. They may have different or similar sexes. Studies also show that DZ twinning talents are inherited to their offspring (genetic), but only female / female offspring are able to show it (because only females / females can manage ovum release).
The term twin dampit is given to twins with different genitals. The terms twin mixes are given for twins with different skin colors.
Monozygotic or identical twins (MZ)
Monozygotic twins occur when a single egg is fertilized and forms a zygote (monozygotic). In its development, the zygote divides into different embryos. Both embryos develop into fetuses sharing the same uterus. Depending on the stages of zygote separation, identical twins can share the same amnion (known as monoamniotic) or different amnion. Furthermore, identical twins are not monoamniotics can share the same placenta (known as monochorionic, monochorionic) or not. All monoamniotic twins must be monochorionic. Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. For example, the umbilical cord of monoamniotic twins can be entangled thus reducing or impairing blood supply to the developing fetus.
MZ twins are always the same sex and are genetically the same (clones) except when mutations occur in the development of an individual. The twin level of similarity is very high, with differences occurring in the form of mirror similarities. Differences occur in terms of detail, such as fingerprints. When an individual grows up, the level of similarity is usually diminished due to personal experiences or different lifestyles. The study of Fraga et al. (2005) revealed the effects of epigenetics in the differentiating process
individuals who are MZ twins, due to the different genes that are activated. Despite the influence of habits or experiences that affect those differences, scientists assume that random processes play a greater role in the differences that occur. Studies with mice even showed a difference in activity on histones (associated with epigenetics) of the first four cells formed.
Until now scientists have not agreed on the existence of genetic influences for the incidence of MZ twins. But it is known there are some places in the world that have MZ twin frequencies higher than elsewhere.