Twins are defined as two babies born during the same birth, which means they cannot be born years apart. Generally, twins are born within a few minutes to an hour of each other.

Through the years, medical studies have shown that the time between the birth of the first twin and the birth of the second twin can be a risk factor for the second baby. According to studies published by the National Institutes of Health, the interval should be kept short when possible. In some cases, the premature birth of one twin results in a long delay before the birth of a second. In 2012, an interval of 87 days occurred between the birth of Irish twins for this reason.