About 1 in 26 births are twins so unless you have undergone fertility treatments, which do increase those odds, it is still a long shot that you will have twins. Of the twins born in this country, 75% are fraternal and 25% are identical. Fraternal twins tend to run in families and various fertility drugs also increase your chances of having them. Fraternal twins may or may not be of the same sex.
There are no known causes for identical or one-egg twins. It just happens. These twins are always the same sex, the same size and often they have very similar temperaments. Strangers will have a hard time telling them apart. And, although they may look and act alike, they are two separate, distinct human beings and should be treated as such.
At birth, your doctor can usually determine whether you have fraternal or identical twins, but not always. You will know for sure after a few months. They will either continue to look identical, or they'll develop important physical differences in appearance.
Twins are twice as much work, but also twice as much fun. Oh, and if you think you may have triplets count yourself in a real minority: the odds of having three chilren in one birth are 1 in 7,000!