There are two kinds of twins. The first is called monozygotic twin (MZ), the twins who came from the same egg. This is often called identical twins, although this time new evidence emerged that doubted the identical-an MZ. The second is called dizygotic twin (DZ), the twin derived from egg cells are different.

The exact cause of multiple pregnancies remains a mystery. However, many studies indicate factors that contribute to the occurrence of multiple pregnancies. These factors differ between MZ and DZ twins.

MZ twins are more likely to be viewed as a random occurrence (coincidence) in the population. However, some studies suggest a grouping of families (family clustering) on the incidence of MZ. This indicates the existence of genetic factors that play a role in causing the MZ twin pregnancies. Genetic factors are still not able to be identified with certainty.

DZ twins in principle occur due to discharge more than one egg in a single ovulation. So in this case the mother is very dominant factor. Factors which contribute to the incidence of DZ twins among others, maternal age at delivery. Research shows that the older the age gave birth to twins, the incidence is increasing.

However, it should also be noted that the old age of childbirth is one risk factor for congenital defects, high body mass index and height (related to hormones that affect ovulation), history of twins in the family, and ethnicity (the highest is Nigeria, with an incidence of twin 1 in 11 births, and lowest in Japanese men with the incidence of twin pregnancies and 1 in 250 people of Indonesia seem to be in the middle.

Some foods that contain estrogen and high insulin growth factor, such as milk, eggs, dates and many others also have been studied served to increase the likelihood of multiple pregnancies.

Although not much evidence to show the role of fathers in the incidence of twins, but recent research shows that the proportion of men with children who have a twin brother (far or near) which is also the father of twins is high enough.

This means that hereditary factors also exist in male twins. In addition, DZ twins are not only requires the ability of mothers to produce more than one egg at a single ovulation cycle, but also the ability of fathers to produce sperm cells are able to fertilize more than one egg.