Causes of Fraternal Twinning
It is felt that fraternal twins are conceived due to higher levels of Follicle Stimulating Hormone (FSH) in the mother. This hormone stimulates the growth and ripening of eggs. Mothers of fraternal twins tend to have increased height and weight, earlier start of the menstrual cycle, and shorter menstrual cycles, all of which are probably hormone-related.
Once you have had fraternal twins, your chance of having another set of twins is three to four times that of the general population.
Often, there are other female relatives who have conceived twins as well. Therefore, this tendency to have increased FSH, and as a result fraternal twins, is thought to be inherited. Also, as women age, their FSH naturally rises. Therefore, the incidence of fraternal twins is very common in older moms, especially over the age of 35.
The ability to conceive fraternal twins has ethnic differences also. Fraternal twinning is most common among African-Americans and less common among Asian populations.
Causes of Identical Twinning
It is not really known how identical twinning occurs, but there are some theories. One theory is that it may be related to aging of the egg after ovulation: an "over-ripe" egg. Another theory states that since the rate of identical twins is higher in mothers who are either very young or very old, identical twins may be due to the hormonal imbalances seen at these ages. Supporting this, mothers of identical twins have been shown to have an earlier menopause, which is also due to hormones.
The rate of identical twinning is fairly constant in different countries and ethnicities, unlike the rate of fraternal twinning which is very different in different cultures and races.
Some new research has shown a "male factor" that may contribute to the rate of identical twinning. Identical twinning has been found to run in some families through the male line of the family.
Having two sets of identical twins would be a rare occurrence. The odds of having more than one set of identical twins are at least one in 70,000.