Twin births are rare, but not extremely rare. In the United States, perhaps three in every one hundred pregnancies will result in a multiple birth. That doesn't mean that your odds are three in a hundred. Your chances may be higher or lower, depending on your particular circumstances, and will only be an approximation. To estimate your odds, you should know more about twins.
Kinds of twins
Basically, there are two different kinds of twins. Identical, or monozygotic twins, come from one fertilized egg that has split into two embryos very early in the pregnancy (probably less than 8 days or so after conception). These twins have the same gender, the same blood type, and tend to look alike, although they don't necessarily look identical.
Fraternal, or dizygotic, twins are born from two separate ova that were fertilized by two separate sperm. They may be different genders, have different blood types, and look as alike or as dissimilar as any two siblings can. Genetically, they are siblings who happened to be born at the same time.
Monozygotic, identical twins, appear to occur at about the same rate everywhere in the world that they have been studied. They happen at a rate of about three per thousand births, according to most authorities, regardless of the characteristics or circumstances of the parents. The chances of having dizygotic, fraternal, twins on the other hand, vary widely.
Factors Affecting Fraternal Twinning
The average worldwide occurrence of fraternal twins may be about 12 in 1,000 births. This is an estimate, because not all groups have been studied, and because the rate of multiple births is changing. In fact, it is rising sharply. More multiple births are occurring partly because of the use of fertility-enhancing methods. Fertility drugs and in vitro fertilization promote multiple births. Also many women are choosing to have children later in life, which also promotes multiple births.
Ethnicity also plays a role in the chance of having fraternal twins. The women of the Yoruba tribe are believed to have twins at a rate of 45 per 1,000 live births. Some other African groups also have high rates of fraternal twin births. The average of fraternal twin births for Japanese women is believed to be about 1.3 per 1000 births. In California, something like 10 fraternal twin births per 1000 occur to Caucasian women.
Heredity plays a strong role in the probability of having fraternal twins. The tendency for the ovaries to produce more than one egg per cycle is inherited in some families. It can be inherited on either the male or female side, but will not result in the birth of twins in the current generation if it is inherited through the father, because he has no ovaries. His female children, though, may have an increased tendency to have ovaries that produce extra eggs.
It is extra ova which result in dizygotic twins. Besides heredity and fertility treatments, other factors associated with a tendency to produce extra ova and thus have twins are increased maternal age, increased maternal body weight, several previous pregnancies, and extreme maternal height.
Mothers who have already had a set of fraternal twins have a four times higher chance of having more if they become pregnant. If the mother or sister of the person carrying a child had fraternal twins, the possibility of her giving birth to twins is doubled. Some authorities think that having children after 35 also doubles the odds of having twins, but this is generally doubling a very small number.
Summing it up
It is impossible to calculate your exact probability of having twins. Science has not advanced that far. However, you can make an estimate, based on your age, your ethnicity, and your family history. If you are a white person living in the US, with no fraternal twins in your family, and are under 35 and not extremely tall or overweight, your chance of having fraternal twins may be about 1.5 %. That is around the national average.
If you have fertility treatments or are of African descent your odds improve. If you are from Japan, your chance of having twins probably declines. Whatever your personal circumstances, multiple births are rare. Yet almost anything is possible. Probability can tell you what will probably happen to groups of people in your circumstances. It cannot say exactly what will happen to you.