In most cases of multiple births, two or three are produced and they are known as twins or triplets. These types of births occur in many animal species as well as human beings but usually placental species are known for this term.

Multiple births brothers or sisters formed because of two types of genetic reactions. Call one monozygotic and other di-zygotic. When one egg is split into two or more zygotes, with each carrying the same genetic material, the fertilization occurs and this can produce siblings who will be identical. Since, the identical twins, triples or multiple birth siblings carry the same egg, they look almost same or in most cases, the gender is also the same. Those who are identical in every way but not sex have a reason.

The fertilized egg had an extra chromosome in this case and these fertilized eggs can be of zygotes XXX, XXY or XYY. For example, when a zygote with XYY, when they split, they will have XY and YY. So, the multiple births will be genetically identical but not by gender-wise. This identical case is known as monozygotic.

In case of di-zygotic, number of ovaries either two or more are ripened with sperms so different fetus are created in the same womb and in the same period by the completion of menstrual cycle. When two ovaries are ripened, they are called di-zygotic and when more than that are ripened, they are called poly-zygotic.
These cases are normally not much probable but twins are quiet common in human as well as animal species or say in all placental species.