Identical twins develop from one fertilized egg that splits into two parts, while fraternal twins are born from two different eggs and sperm cells. In both cases, the twins share the same womb and are usually born within moments of each other.

Identical twins are born when a single egg or ovum encounters a sperm cell, which causes the egg to divide during fertilization. When this occurs, two separate embryos are formed. However, the embryos share the same genetic information. Identical twins may also share the same amniotic sac, placenta and other structures. When born, such twins tend to resemble each other quite closely.

On the other hand, fraternal twins are born when a woman produces two separate eggs that are both fertilized by different sperm cells. These twins tend to differ in looks, genetic information and may even be male and female.

One factor which may influence the birth of twins is the age of the mother. Women who are over the age of 30 are more likely to give birth to twins. Women who have had a higher number of previous pregnancies are also more likely to give birth to twins. Other factors include heredity and race, with black African women having the highest odds of giving birth to twins while their Asian counterparts are the least likely to give birth to twins.