There are things that can increase your chances of having a multiple birth, including a mother being 30 or over when she conceives. For example, while there is a 3% chance of having a multiple birth at age 25-29, it increases to 4% at 30-34 years and almost 5% at 35-39 years.
Other factors that can increase your chances of having twins include having a maternal family history of multiple births (fraternal), and of course, using fertility treatments (in vitro fertilization or fertility drugs).
Your chance of having twins is also influenced by your race (multiples are more common in African Americans and least common in Hispanics and Asians), how many times you have been pregnant (the chance of having twins increases with each pregnancy), and whether or not you have had twins already.
Keep in mind that statistics for birth rates and multiple births include all births, including those that were the result of fertility treatments and those that were spontaneous or 'natural'. So if you don't have any of the factors mentioned above that can increase your chance of having multiples, your chance is going to be a little less than 3%. The chance of having 'natural' fraternal twins is only about 1.7% or 1 in 60. The chances of having twins with the use of fertility treatments can be as high as 20-25%!
Of multiple births, the rate of fraternal twins has been increasing the most, and that makes sense since the chance of having fraternal twins is influenced by an advanced maternal age and the use of fertility treatments.
The chance of having identical twins has remained steady, and is about 0.4% or 1 in 250.