What are the odds of having twins?

By Peg Plumbo 
From ParentsPlace.com

Twins can either originate from two separate ova (fraternal) or result from the division of one fertilized ovum (identical twins). Identical twinning (monozygotic twins) is the result of chance and the frequency is fairly constant throughout the world at a rate of approximately four per thousand births.

Fraternal twinning (dizygotic twins) is associated with multiple ovulation and its frequency varies between races and within countries and is affected by several factors. In general, the frequency of dizygotic twins is low in Asian races, intermediate in whites and high in black races. In the U.S., the overall incidence of twins is approximately 12 per 1000 and two-thirds of these are dizygotic.

The chance of having fraternal twins goes up with increasing maternal age. Three woman in 1000, under the age of 20, will conceive fraternal twins. Between the ages of 35 and 40, 14 women in one thousand will conceive fraternal twins. Women who have had a greater number of babies (increasing parity) also have a higher risk.

It is the fraternal type of twinning that is induced by fertility drugs. Although the rates differ by type of treatment, twinning can occur in over 20 percent of the conceptions when some fertility drugs are used to induce ovulation.