Offsprings delivered from two embryos that have developed simultaneously in the same uterus are known as twins. Twins are of two kinds, monozygotic and dizygotic twins. Twins formed from a single embryo are known as Monozygotic twins. During early times of the embryonic development, that is within eight days of zygote formation, the cells of the embryo are divided into two groups, which develop as two separate embryos and this results in formation of monozygotic twins. These twins are genetically exactly similar to each other, that they are exactly similar in appearance and their gender is the same, both will either be girls or boys. If the embryonic cells are divided into two groups eight days after the zygote formation, there is possibility of conjoined twins (Siamese twins). These twins have some parts of the body joined to each other. They may also have some common organs. Meanwhile, sometimes two
oocytes are released from the ovary of a woman and these oocytes are fertilized by two separate sperms and form two zygotes. These two embryos are formed from those two zygotes and both of those embryos are separately implanted in the uterus and thus dizygotic twins are delivered after complete development. Such twins are genetically different and may be the same or different by gender.