The probability of identical twins is very low and not very high. The probability of twins occurring is significantly lower than the probability of singletons occurring, while the probability of identical twins is very low and not very high. Twins can be categorized into identical and fraternal twins. Identical twins refer to the gradual division of a fertilized egg to form two embryos. Dissimilar twins refer to the formation of two embryos from two separate fertilized eggs. Identical twins are more similar in physical characteristics, external morphology, intelligence and blood type. If identical twins develop abnormally, deformities such as conjoined twins may be formed.