Twins are two people who have shared the same womb at the same time. They may also have come from the same egg. Twin births are becoming more and more common due to several factors such as increasing use of fertility treatments, child births at an older age, and even genetic factors. Women who are slightly above-average in build are also likely to have multiple births or twins. Certain ethnic groups are also found to have a greater twin population. One in every 250 births in the world could be an identical twin birth.