There are a number of factors that may increase the likelihood of a person conceiving twins without the assistance of fertility treatments. These can range from a person’s family history and race or geographical area to the person’s age, weight and height.
A family history of multiple pregnancies can increase the likelihood of someone conceiving multiples. Genetics can make someone more likely to release more than one egg, resulting in the conception of twins, says Dr. Cackovic. However, he notes that this “likely runs in families on the maternal side as opposed to the paternal side.”
There are also certain parts of the world where twins are more common than others. For instance, the rate of twins in Japan is only 1.3 in 1,000 births. Meanwhile, in Africa, particularly, Nigeria, which has the highest incidence of twins in the world, the rate is significantly higher. Among the Yoruba people in South-West Nigeria, a rate of 49 to 53 sets of twins per 1,000 births has been reported.
A correlation between race and the incidence of twins has also been noted. Twins are most common among Black people, who have the highest likelihood of having twins of any race. The occurrence of twins is lowest among Asian and Native American people, while caucasian people, particularly those over the age of 35, tend to have more higher-order births, which includes triplets and higher-order multiples.
The older a pregnant person is, the likelier they may be to have twins. “The risk of twins actually increases when a woman gets older, especially over the age of 30 or 35,” says Mindy Christianson, M.D., medical director of the Johns Hopkins Fertility Center. “This is because certain hormones can increase, which increases the risk of releasing more than one egg.”
People who are taller or have a higher body weight also tend to have a higher likelihood of conceiving twins. According to Dr. Christinanson, this may be due to hormonal differences related to body composition. For instance, having a higher level of body fat can lead to increased levels of circulating estrogen, which can increase the likelihood of releasing more than one egg during ovulation, in turn increasing the chances of conceiving twins.