There is no way to ensure you have twins.  he likelihood of having identical twins is about one set per 250 births.  Fraternal twins however are said to be influenced by race, heredity, maternal age and number of previous pregnancies and are even more common due to fertility drugs.

If you are a twin or twins run in your family you are slightly more likely to conceive twins yourself.  Women who are fraternal twins have one in 60 chance of conceiving twins themselves.

Apparently, the older you are the higher chance you have of conceiving fraternal twins.  A study found that women over 35 produce more follicle stimulating hormones than younger women.  The more pregnancies you’ve had the greater the chances of having twins.  If you have already had a set of fraternal twins you are twice as likely to have another set in future pregnancies.  If the firstborn are twins you are about five times more likely to repeat twinning.

Twins are more common in large and tall women.

Fertility treatment increases the odds of twins as the drugs administered stimulate the ovaries increasing the odds that a woman will release several eggs at the same time.  Approximately 20% of women who fall pregnant using fertility drugs will become pregnant with more than one baby.  Fertility drugs and other treatments primarily increase the chances of having fraternal twins, where two eggs are fertilized by two different sperm.  However, there is some evidence that women who undergo certain treatments also have a higher rate of identical twins.

Identical Twins are described as “spontaneous” and somewhat of a mystery.