How are twins born?

Twins are two offspring created by the same pregnancy. There are two types of twins, identical or monozygotic and non-identical or fraternal.
In monozygotic twins, one egg is fertilized by one sperm cell and the zygote formed as a result splits to form two embryos. Monogyzotic twins share the same genetic information. They also share the placenta that provides a link to their mother's body.
Fraternal twins on the other hand are born when two different eggs created by the mother are fertilized by two different sperm cells at the same time. Each fraternal twin has a unique placenta. Fraternal twins are like two siblings born at the same time. They do not share any genetic components and like siblings may or may not resemble each other.
The birth of twins is similar to that of singletons. Once the babies have developed completely, the mother has contractions which pushes them out of the womb.